How to line up your mixer and recorder for improved recordings.
Last month we gave you a circuit for a 1kHz test tone oscillator, this month we thought we'd tell you how you should use it...
A test tone is used to line up a tape machine for the purpose of recording a sound and playing it back at the same level. Many sound mixers have a fixed frequency test tone oscillator built in, the frequencies normally being 100Hz, 1kHz and 10kHz, though many provide only the 1kHz frequency.
With the growing popularity of home recording and the equally expanding range of equipment, new problems are appearing. Many semi-pro Japanese mixers and tape recorders operate at a different level (-10 dBm) to professional equipment and therefore use different reference frequencies, normally 400Hz. Some time ago the Telephone and Broadcasting industry adopted a standard reference level for the calibration of sound recording and transmitting equipment. On a VU meter the zero level is specified as 0dBm = 0.775 volts.
With the advent of high level tape, many manufacturers have increased the operating levels of mixers and tape recorders and so the 'zero level' has been changed to a new operating level of +4dBm. This is now the norm for all professional equipment and much of the semi-pro gear as well.
Predictably, the existence of three different operating levels has started to cause problems: those people who prefer to choose equipment for it's individual merits rather than sticking with one manufacturer's products have found difficulty in linking everything together. A mixer with a +4dBm output will not operate properly into a tape recorder with a -10dBm input. This is essentially an interface problem and can be easily overcome by attenuating the signal levels coming from the mixer. On tape recorders with input level controls there's no problem, however, on equipment with fixed inputs, eg. the Tascam 38, you will need to purchase or build an attenuator to drop the +4dBm signals down to -10dBm.
Once you have matched the levels of all your equipment you will then need to calibrate everything prior to starting any recording.
Switch on the mixer's oscillator, select the 1kHz frequency and adjust the oscillator output level to read 0 VU on the meters. Patch the output of the mixer into your tape or cassette recorder, and adjust their input record levels to read 0 VU as well. You have now calibrated the mixer and recorder. Easy isn't it? You now only have to watch one set of meters; those on the mixer.
Having done this record the oscillator 1kHz tone onto the tape. If you now replay the recorded tone off tape, it should register as 0 VU again. This means that every sound that goes onto the tape comes off it at the same level.
You can imagine the problem you would face if all the tape machines you were using were in different parts of the room, each with a different level setting. It would be impossible to keep an eye on them all, therefore it makes sense to calibrate each machine to the same level as the mixer. This will then allow you to concentrate on the mixer VU meters whilst recording, which also means that you are sitting in the right position during the mixdown ie. at the mixer and between the monitor speakers.
Other than checking record and playback levels, a test tone will help you or other people in possession of your finished tape to line up their machine for the correct reproduction of the music. This is particularly important for cassette copying and disc cutting purposes, or when submitting demo tapes to record companies.
As well as the 1kHz tone, many-studios record 100Hz and 10kHz frequencies onto the tape. This enables the tape engineer to quickly check the playback levels at three points across the audio range of the tape. The 100Hz tone is to check bass frequency response, whereas the 10kHz tone would soon show up any misalignment of the playback head, as well as any high frequency loss.