Symmetric microphone lines have the advantage that electromagnetic noise (usually in form of 50/60 Hz hum) is not induced and therefore you may safely use long microphone lines. If you intend to use high quality, symmetric studio microphones in combination with a computer as the recording device, consider using a semi-professional mixer between microphones and computer. Most mixers have symmetric inputs and (at least some) asymmetric outputs that may be connected to standard sound cards. If your budget allows the additional costs you may also use a digital mixer and a digital sound card that can be connected directly to the digital output of the mixer. This has also the advantage that you can record up to 16 channels in parallel into one computer.
Whatever microphones you use (low or high quality) try to set them up in
way that the average speech level will be within the lower 40% of your
dynamic range. For example: Using 16 bit samples the average speech level
should not exceed
. Or set the maximum sound
pressure to - 12 dB. Test this with a speaker
with a very loud voice. Add the settings of the amplifier to your check
list; do not change the settings during one recording session, and if you must
alter it, document this in your recording protocol.
Also add a check point that batteries of amplifiers are
checked before every recording session. Always keep fresh batteries
available. If you use AC power amplifiers, make sure there is no 50/60Hz
hum on your signals. Beware: it is not sufficient to simply listen to the recordings to
check the signal quality because some D/A cards or headphones might not
reproduce low
frequencies; it is much better to check a sonagram or calculate some spectra.