|Paul Schmidt's Recording Page:
Digital Audio Recording Primer
|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.
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.
Here are some tips for selecting and using various audio recording equipment.
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.
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:
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.
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...
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.
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.
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.
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.
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||Bogen stands|
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Copyright Paul Schmidt 2002