Paul Schmidt's Recording Page:
Digital Audio Recording Primer
 

 



 
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Background  I  have been making quality audio recordings of live concerts for about 20 years. I first began with groups that I played or sang in, setting up some home audio recording equipment before concerts so that the other performers and I could hear what it sounded like afterwards. This led to requests from other groups, and pretty soon my services were expected by default. 

I began with a stereo cassette deck and a couple of Radio Shack microphones on cheap Radio Shack stands, and I made copies one at a time by dubbing between two cassette decks. These recordings still sound decent, but the fidelity is not up to current standards and expectations.

Next, I got better semi-pro mics from a high-end audio store, and switched to open reel tape in place of cassettes. I soon discovered that the open reel tapes had more headroom, better frequency response, and less pitch distortion; however, they had more hiss. I added DBX noise reduction and the recordings started to sound rather professional. Copies were still on cassette, but a double speed duplicator deck made the the task of producing copies less of a chore.

With the higher fidelity, I started to notice some hum in the recordings, and discovered this was due to power line noise that was being picked up by the mic cables. I learned that professionals use 'balanced lines' between the mics and recording equipment to prevent hum. I got some appropriate cable, added the corresponding 'XLR' connectors, and bought balancing transformers to use at each end of the cable. This reduced the hum but added some 'coloration' to the sound, since the transformers do not have a uniform frequency response. I once again invested in better mics, this time low end professional types with transformerless balanced outputs. I also got a small Heathkit audio mixer with built-in mic preamplifiers and balanced inputs. When this unit's preamps turned out to be a bit hissy, I replaced it with a semi-professional Marantz portable mixing board with transformerless balanced inputs and quieter inputs. Copies were still on cassette.

The early Sony 'F1' series PCM digital recorders came out, allowing CD quality stereo recordings to be made using video tape as the media (the digital information was encoded into a video picture of rapidly changing bar-code). I upgraded to this and finally had really clean recordings, with one problem...the mics, whose internal noise had been previously masked by tape noise, became a limiting factor. Another upgrade left me with professional mics with extremely low noise and ruler-flat frequency response. The pin-drop quiet mics and recorder now revealed the mixer's preamp as the remaining noise source. Only top quality professional preamps would do the trick, so I took the plunge. Everything was perfect.......but NOW the poor quality of the cassette started to bother me. I discovered that many tricks are used by cassette decks to simulate high fidelity audio, something for which the media was never designed....these tricks can fool the ear if the audio material has lots of it's own noise and other distortions, but with a clean and quiet digital source, the cassette tricks quickly became audible and undesirable.

Purchasing one of the first stand alone CD recorders took care of the cassette  problems. However, it was now too expensive to edit to a blank CD, as even the smallest mistake would ruin the disk. I obtained a DAT (digital audio tape) recorder, which allowed me to make clean edits from the PCM masters to DAT tape, which could them be copied without loss or distortion to the CD recorder. But now there was another problem...everybody now wanted CD copies instead of cassettes, and making copies one at a time on the recorder was very time consuming. I had to buy a CD duplicating 'tower', allowing me to make multiple CD copies at once from the master CD, and they could also be done at high speed!

More problems....the PCM-to-video format was obsolete, and the slow video tape drives caused the editing process to be tedious. I upgraded from PCM to DAT recorders for making the original field recording. The equipment was now less expensive, making multiple decks affordable, which in turn provided redundancy for both the recorders and the master tapes.

This web page is intended to provide basic information on the equipment required to do professional quality digital recordings with minimal expense, and incorporates a number of points that I have learned through the above evolutionary process.

The text is geared towards the recording of live concerts and recitals, and is less than applicable to other kinds of setup such as amplified pop music, sound reinforcement, public address, etc.



 
Equipment  Microphones:
The sound will be no better than the microphones. For most live recording in concert situations, 'condenser' mics with a 'cardioid' pickup pattern are best. They should be low impedance with balanced outputs, should be equipped with some sort of shock isolation mounting, and be positioned well off the floor on suitable stands. Most 'classical' concerts are served well by a single pair of such mics, arranged in a 90 degree crossed pattern and positioned on a single 'tower' stand high in the air and fairly close to the performers. No mixer is required for this setup, and a very natural stereo image is obtained. Semi-pro condenser mics will require a battery to power their internal circuitry, but professional models will get their power from the preamplifier via the mic cable in the so-called 'phantom' scheme; some mics can use batteries or phantom power.

Cables:
Cheap mic cable will spoil the best program. Invest in quality cable of the kind intended for this purpose, take care of it (always roll it up on reels to prevent twisting, avoid pulling on it to avoid stretching, and make sure that it is not kinked, pinched or crushed. It will last a lifetime. Buy a long length, and cut it into equal shorter lengths, with extra XLR connectors; this is flexible because it accomodates sessions where any length of cable might be required (you will be able to use two or more short lengths, or chain them together for longer cable runs).

Preamplifer:
All mics need amplification before they can be used for recording. An excellent preamp will last practically forever, so invest in a good one that will not let you down later. Preamps of this kind are most readily found in professional mixing boards, but can be obtained as separate units. Most professional mics with balanced outputs use 'phantom' power; the preamp supplies power to the mic via the mic cable, using the same wires as the audio signal. A preamp's phantom power should be defeatable, so that battery powered mics can be used. The output signal of a preamp will be 'line' level, and will be either 'single ended' (unbalanced) or 'differential' (balanced), or both. Single ended line level signals can be applied directly to the audio inputs of any recording device ranging from home stereo components to professional equipment (the 'RCA' style connectors are usually used for this type of signal, although 1/4" diameter 'phone' types are also used). As long as the preamp is going to be close to the recorder (preferably next to each other), there is no benefit to using balanced cables between the preamp and the recorder; this allows less expensive 'consumer/home audio' or semi-pro recorders to be used (in many cases the only difference between the sound quality of consumer and professional equipment is the use of balanced versus unbalanced inputs and outputs.

Recorder:
The best all-around digital recorder is the DAT type. PCM is pretty much obsolete, MP3 and Mini-Disc are compromised formats that are unsuitable for professional quality recordings, recordable CD (audio CD-R) is nice but the decks are more expensive, the recording length is much less, and they are not practical for editing. Hard-drive (computer based) recording is an option, but is too expensive for most people.

Editing:
If the concert is sufficiently planned and coordinated, it is possible to do fade-ins and fade-outs, and pause the recorder between selections; this makes the master tape suitable for use as an edited master, which can in turn be used for making copie(s). If you can always count on this being the case, you don't need any other editing equipment. In all other cases, some sort of  editing equipment is a must, and can include another DAT recorder, a computer equipped with a serious sound card and audio editing software, or a stand-alone CD recorder.

Duplicating:
Few people will be able to listen to your hard drive or DAT recording, so copies should be on CD. It is easy to make audio CD recordings these days, but without a duplicator, making more than a few will be very time consuming. CD duplicators are usually stand-alone types with two or more CD drives and a built in computer to control them. It is also possible to put multiple drives into a case with a power supply, and control them from a regular personal computer; for this to work, special software is required, and few companies make this kind of software. With the cost of ready-to-go stand-alone duplicators falling, these are probably the best bet.

Media:
Never get cheap with the media used for the master recording; this is not the place to save money. Also, DAT tape should ideally be used once only (don't re-record on a tape intended for mastering), but the tape used for editing might be used several times. With recordable CDs, keep in mind that much blank CD-R media is produced for computer data storage, not music; as such, it's real life span might be less than 10 years! Use the more professional blank CDs with dies that will last 100 years of better.
 



 
About Components

Here are some tips for selecting and using various audio recording equipment. 
 

Microphones >

Here are three mics that I have found very useful; similar mics are available. At left is a good consumer/semi-pro mic by Teledyne; it takes a single AA battery and has a transformerless balanced low impedance output. Less than $100.
At center is an inexpensive professional mic by Audio-Technica that can use an AA battery or phantom power; it has a transformerless balanced low impedance output. Windscreen cannot be removed. Around $300. 
At right is a very nice professional general purpose recording mic by AKG, priced in the high hundreds; very similar to the center mic in performace, but frequency response is flatter and noise is inaudible. It uses phantom power only.

All such mics should be attached to stands using shock mounts. They all have XLR type connectors. Condenser mics such as these are very sensitive to air movement; for use close to performers that are moving, or near open windows, under ceiling fans, or outdoors, they must have windscreens. It does not take much air motion past the mic to 'blow it out', rendering it temporarily useless until the charge has time to build up between the electrical plates of it's pickup element. Condenser mics are not so good for use by vocalists (not good for handheld use and for being close to the air coming from the mouth).


< Mic mounts and stands

An amazing amount of vibration can be picked up from the floor, and the mic stand itself can pick up sound waves. The mic will react to both, with undesired results. Shock mounts, such as the pictured one by Audio Technica, are around $30-$50 each and are well worth the investment.

A few mic terms:
Condenser - An electronic circuit in the mic places an electrical charge across two metal plates, forming a capacitor ('condenser' is another name for this). The air pressure from the sound wave make minute changes in the distance between the plates, changing their capacitance and charge characteristics. The mic's circuitry senses these changes and converts them to an audio signal. Condenser mics are quite sensitive (good for high fidelity), tolerant of wide extremes in sound level (good for close or distant pickup of instruments), and they have very flat frequency response (they don't color the sound).
Cardioid - The mic is most sensitive to sound coming from the front, with sensitivity being progressively less towards the rear. The pickup pattern looks like a heart, hence the name cardioid. Used in an appropriate orientation, cardioid mics give an excellent stereo image while cancelling most audience noise.
Balanced - All that is required to send an audio signal is a pair of wires; the signal goes out in one wire and comes back on the other wire, completing the circuit. However, magnetic fields near the cable will induce noise signals into the wires, and the preamp has no way of distinguishing the good signal from the unwanted noise. With balance cables, the cable has three wires...the extra wire carries an inverse, or mirror image of the signal on the other wire going to the preamp. Any induced noise is added to the first signal and subtracted from the mirror signal. The preamp looks at the difference (differential) between the two signals, which allows it to isolate the good signal and reject the noise.
Phantom Power - The mic sends oscillating (AC) current through the mic cable to the preamp, while the preamp sends non-oscillating (DC) current the other way to the mic. The same wires are used for both signals. Only mics that are connected via balanced cables can use phantom power.
 

Stereo pair >

The classic mic arrangement for recording a a live concert in stereo. Two mics are positioned close together, usually less than a foot apart, with a 90 degree angle between them. For this to work, the mics must have a cardioid pickup pattern. It is a good rule of thumb to locate the single mic stand about as far back from the group as the group is wide; too far back and the stereo image will be less distinct, and too close will result in a 'hole' in the middle of the stereo image. A quick and dirty test for the group-to-mic distance is to record yourself walking across the stage, etc; while talking at a constant volume. Upon listening to the recording, you should be able to clearly tell exactly where the sound is coming from as it pans from one side to the other; if the sound changes volume or the position is not distinct, reposition the mics.

< Balanced mic cable

Good balanced mic cable has two wires surrounded by a shield, wrapped in turn with a cloth weave and surrounded by a durable rubber jacket. The two wires ('conductors') will be twisted, the shield will be a braided type with tinned copper 'threads'. The shield is there primarily to reduce pickup of radio signals and other high frequency noise. The twisting of the two inner conductors reduces pickup of magnetic fields; any magnetically induced noise that is picked up in spite of the twist will be cancelled by the balanced preamp circuitry. These two conductors carry the audio signal and its 'mirror' signal, and they both also carry the DC phantom power on its way to the mic. The shield acts as the electrical common (but it carries no current associated with the audio signal), and it also carries return current from the phantom power. If the cable is to be used in environments where there is not much magnetic field noise, cables with untwisted inner conductors may be suitable. It is important to have a good quality braided shield with no visible gaps between strands. Cheap cables might have an unbraided shield, which will be pretty much useless. Some mic cable will have a metal foil shield instead of a braided wire shield; these are actually superior for cable that will never see much flexing (as in permanent installations such as a recording studio), but foil shields fail quickly if the cable is to be flexed or rolled.
 

 

XLR connectors >

Balanced cables, whether used for mics or for other audio applications, use a standard connector called 'XLR', or sometimes 'Canon'. Some older and/or cheaper equipment uses the three conductor version of 1/4" 'phone' connector, the same kind used for stereo headphones. The twisted inner conductors use pins 1 & 2 on the XLR, or the tip and ring of a phone connector (whichever wire goes from pin 1 / tip at one end must go to pin 1 / tip at the other end, ditto for the other wire at pin 2 / ring). At the very least, the shield needs to connect pin 1 / shield ring, and many cable manufacturers also connect the XLR connector's metal shell to pin 1; on all cables I have looked at or made, this is done only at the 'female' end of the cable - it has nothing to do with shielding the cable itself. The female end is the where the signal enters the cable (e.g. at the mic end); if the connector shell is connected to pin 1 here, the mic's metal case will be effectively connected to the cable shield.


< Connector types

Here are the most common types of connectors used for quality audio equipment, along with their proper names.

Left: 'RCA / Phono' type plug (male) and jack (female), Used for most consumer grade audio connections, as well as being an option on semi-pro and professional equipment. When used with semi-pro or professional gear, this connector type almost always imples single-ended / unbalanced 'line level' audio signals.
Center: 'Phone' type plugs, in 1/8" (left) and 1/4" (right) sizes. The 1/8" plug shown is a stereo type and is not commonly commonly used for professional equipment. The 1/4" connector is usually used for professional single ended line level signals, while the three conductor / stereo version (not shown) is used as a less expensive and/or smaller diameter alternate to the XLR connector.
Right: 'XLR' female connectors, both cable and chassis versions.

A word on single-ended / unbalanced and RCA style connections: The sole reason for using balanced cables and equipment is the reduction of electrical noise where signals must be run for long distances and/or through electrically noisy environments. The extra circuitry required for balancing adds it's own small amount of noise and other distortions. If the relatively powerful 'line level' signals are to be passed between adjacent equipment, there is no reason to use balanced cable; a short single-ended connection will contribute less noise to the signal and will cost much less. Much professional equipment is identical to lower priced semi-pro and high end consumer equipment except for the use of balanced connections. Much money may be saved in this area by knowing when balanced is required and when it is not.
 

 

Microphone mixer / preamplifier >

For those wanting to make quality live recordings on a budget, nothing beats the inexpensive mini mixing boards made by manufacturers such as Mackie Designs. The photo shows the Mackie 1202 (12 inputs, 2 outputs for stereo), a real classic. It is the size of a notebook computer, is very rugged, four of the 12 inputs have integral balanced mic preamps with excellent performance characteristics, and outputs include single ended line level RCA types and balanced line level 1/4" phone types. Each input has its own mini equalizer, pan control and volume level control; the four mic inputs also have gain controls. Mic inputs can be by either XLR or 1/4" phone connectors, and defeatable phantom power is provided.

If using such as mixer simply for it's mic preamps, the two pan controls must be set at full counter-clockwise and full clockwise, in order to send the left mic's signal to only the left output and likewise for the right mic. Also, resist the temptation to play with the equalizer; if the mics are good ones, you will only be reducing the quality of the overall audio signal. Another tip: use the mic gain controls to compensate for different types of mics, use the volume level controls to set relative balance when the mics are close to unmatched sound sources, and use the master level control to set the overall signal level going to the recorder.


< Editing equipment

Once a line level stereo signal has been obtained from the preamp and recorded to DAT or other digital media, you will probably want to edit it before making copies. You can use a computer with a professional quality sound card and appropriate software, or you can use stand alone equipment. This photo shows a professional DAT editing deck made by Tascam, with a stand alone professional CD recorder sold by HHB. The DAT deck is good for editing for several reasons: 1) it has a shuttle control that allows quick tape position control, 2) it has an advanced tape path system that prevents loose or damaged tape during editing, and 3) a dedicated button for every function, as opposed to cumbersome multi-use buttons found on less expensive equipment. The CD recorder may be used to do the editing, or it may be digitally synconized to the DAT deck; in this case the DAT deck performs the editing, then it controls the CD recorder to make an exact duplicate of the edited tape on a blank recordable CD. Similar equipment will today cost about the same price as a basic personal computer with a good sound card, CD-R drive and basic music editing software. You can go either way...
 

 

Media >

Choosing the proper media for recording is important. Most DAT tape is used by professionals, and as such most brands are of good quality (the same cannot be said of cassette tapes, for example). On the other hand, much blank recordable CD (CD-R) media is of much lower quality than the user might imagine. Be aware that CD-R media comes in two basic flavors; 'music' type and all others. Consumer grade CD recorders are limited to using the much costlier 'music' blanks, while professional recorders can use any kind, including computer type blanks. When any CD recorder first starts writing to a new blank, it makes test burns at different laser power settings, and tests the burns for optimum playback; once the best setting is determined, it is used for the actual recording. The difference is that the lasers in consumer recorders have a much narrower range of power settings, and cannot write to ('burn') the less costly professional / computer blanks. Also be aware that much of the inexpensive blank CD media sold for computer use has a laser sensitive dye that will degrade within only a few years, and may leave the disk unplayable in as little as 10 years. Try to buy from manufacturers who use the more long lived dyes, and don't be fooled by guarantees alone, it is very easy for them to replace cheap media, but that does not help when your recording is lost!

Blank CD media comes in 74 minute and 80 minute versions. The spacing between the spiral 'tracks' is less on the 80 minutes types. This tighter 'pitch' can be problematic for some CD recorders and players, and is likely to have a greater number of errors in general. Use 74 minutes blanks if possible. Besides the dye type, the metalization layer on disks can be gold or some other metal; gold will last longer.


< Duplication

When it comes time to duplicate the master CD, some sort of CD duplicator is a must. The picture shows an inexpensive personal computer next to a tower case partly filled with CD-R drives. The computer has a SCSI interface and the drives all use this to communicate with the computer. The number of drives is limited by the capacity of the SCSI interface and the license level of the duplication software. A master CD may be copied from one of the CD-R drives to the computer hard drive, then played back from there while all CD-R drives are recording, or one drive can play the master while the remaining drives record. The specialized software is made by Prassi Software, a European company. The more popular option is to use a stand alone duplicating tower; it looks just like this tower but includes it's own built-in computer than controls the drives. The stand alone units used to be more expensive, but these days Prassi is getting hard to deal with (at least for the end user), and the stand alone units are quite competative. A major maker of duplicators is Micro Boards.

If you need to make more than a couple hundred copies, consider using a CD duplication service such as Disc Makers.
 

 

Bulk media >

Blank CD-R media is best obtained from professional audio/video suppliers in bulk packaging. Usually 50 or 100 disks are packaged in 'cakes', with or without a hard plastic case (once you get a few cases, start buying the less expensive case-less media and store it in your old  cases). Besides lower cost, bulk media is more convenient to use, since you don't have to unwrap the cellophane from individual jewel boxes, etc. The same suppliers sell bulk CD jewell cases, disassembled and ready to use.

Bulk CD-R media will usually come without any printing on the top surface; avoid bulk media sold in computer stores, as it usually has lower quality dyes and has undesired printing and manufacturer logos on the top side.

You will probably want to add your own artwork to the finished CDs. The most economical method is to print special CD labels and attach them to the disks. Neato is the leading maker of such labels and related products; they use special paper that will resist shrinking and stretching and forming creases, and special adhesive that will not damage the disk or dry out and fall off. Be cautious about similar looking CD labels from other makers; they are often just normal mailing label paper and adhesive cut to a different shape. They have a good chance of falling off in a few years, quite probably destroying the disk in the process. It is also best to use the special CD label applicator tools to avoid putting them on off-center or with bubbles under them.

Headphones

Use headphones to monitor the signal during recording. The best ones are of the closed design, so that you can hear only the recorded signal and not the live sound from the group. It is not necessary to have fancy headphones for this; all you need to hear is that the signal is clean and that nothing bad is happening - you can otherwise trust the equipment to deliver a good signal.

 


Recording to cassette tape

You might have a nasty surprise when trying to make cassette copies from a digital master you made during live concerts. When playing the cassette tape later, the listener may hear objectionable motor-boat effects or sudden signal drop outs. This can be caused by the unwitting recording of subsonic signals during the concert. The digital recorders are only too happy to record this inaudible stuff, but it will play havoc with the bias signal used by cassette decks when recording. The solution is to obtain a good subsonic filter and place it between the DAT deck or CD player and the cassette recorder. Many consumer graphic equalizers have this feature, and you might find that you already have such a device buried inside some equipment you bought years ago.
 

SCMS

The folks at the record companies have insisted that consumer digital audio equipment be equipped with the Serial Copy Management System (SCMS). Commercial CD recordings will have a digital bit set in their stored code, and this will be detected by a digital recorder and indicates that copyrighted material is being copied. The recorder will make the copy, but will set the bit differently on the copy. On any subsequent attempts to copy the first copy, the equipment will detect the bit and will refuse to make the copy. Professional digital audio equipment usually ignores these bits, and can often be configured to set the bits either way when making a recording. Imagine making a nice digital recording from a live concert...the master tape is the first generation digital copy; when you edit this to another tape or CD, the edited version is a second generation copy and will be allowed. Then, when you try to make copies of the concert for others, the equipment will refuse to do so, thanks to SCMS! You can get around this by edited between decks using the analog cables instead of digital cables, but this can result in more distortion (albeit probably not enough to dustortion to actually hear). The best way to avoid SCMS is to buy professional equipment.
 


Setting levels

Analog tapes decks had VU (recording level) meters that had '0 dB' marks near the center of the scale; you were supposed to record the signal at a level high enough to just go past 0 dB for a bit during the loudest passages. The idea was that the tape could hold a signal up to 0 dB without becoming overloaded, but could manage brief / transient signals that went over 0 dB (such as loud drum beats). You needed to keep the signal as high as possible without going past 0 dB too often or by too large an amount.

Digital recorders have a similar level meter, but it stops at 0 dB. With digital, the sound wave's instantaneous level is converted into a series of numbers, and these numbers are what is stored on the tape. The highest digital number that can be recorded determines the loudest audio signal, and the recorder cannot store anything louder than this, not even by a small amount. 0 dB represents the level at which the highest number is being recorded. The objective is to record at the highest level that comes right up to 0 dB without trying to go past it. Recording at a lower level might prevent slamming into 0 dB, but it might also result in a less accurate recording, since fewer numbers are being used to represent a range of signal levels. Most recorders have a counter that shows the amount of 'headroom' (unused range of levels) on a particular recording. If this number never exceeds 3 dB, the recording should be redone with the recording level set a bit higher (not possible with most live recording sessions). Experience helps!
 



 
Links AKG microphones

Full Compass audio/video

Audio Technica microphones

Tascam

HHB

Mackie mixers

Belden cable

Whirlwind cable

ULine packaging (CD mailers, etc)
 

Bogen stands

König + Meyer (K+M) stands

Benchmark Media Systems mic preamps

Microboards duplicators

Discmakers

Neato labels

Musician's Friend audio/video

Polyline media & packaging
 


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Copyright Paul Schmidt 2002