Tape Guide (Q and A) (Jan. 1976)

Dolby and High Frequency Loss

Q. I have compared "Dolbyized" tapes, using the Dolby system, on playback to the original source from which the tape was recorded. The tape has a muffled sound as compared to the original. Also, much in percussion attacks, higher violin notes, etc., is completely missing from the tape unless I make the tape with the Dolby system switched out.

Moreover, when comparing cassettes made from the same source, the tape which has not received the Dolby process has clarity and high frequency performance, superior to the tape utilizing the Dolby process.

I have a passion for clarity in my music. The muffled sound which accompanies my Dolby system is so frustrating that I'd almost rather put up with the exaggerated tape hiss and switch out the Dolby system when playing my "Dolbyized" cassettes.

-Name Withheld

A. There are factors which could make your Dolby system perform incorrectly. However, when everything is working as it should, the high frequency response from Dolby-processed tapes should be the same as that produced by tapes made with the same recorder, but with the Dolby system switched out of the circuit.

It could be that the system is not properly calibrated. If the Dolby system is supplied with a large signal, no treble boost will take place during recording. If the playback is calibrated incorrectly, however, treble cutting will still take place, ruining the recording. To check this, make a recording. As you do so, alternately switch the Dolby system in and out.

Play the resulting recording with the Dolby system switched out. Note whether you can hear when the Dolby system is switched in and out. If you hear no difference you will know that the Dolby system is either not working at all during recording or that misadjustment of its input is present.

If you find that you do hear a pronounced difference, especially on softer music, during the foregoing tests, something is wrong with the playback. Possibly the Dolby system is not receiving sufficient signal. That would mean that it would always cut treble excessively.

Perhaps you are using the wrong bias or the wrong tape for the bias to which your machine has been set.

This would result in less treble than you have. An error of this kind would possibly not be too serious when listening to a straight playback. The Dolby system, however, magnifies such errors. To check all this, measure the overall frequency response of your recorder, with the Dolby system switched out of the circuit. If it is not flat, make bias and/or other adjustments so that the machine will have as flat a frequency response as possible.

To sum up, whatever happens to the signal during the recording process when employing the Dolby system, is exactly the reverse of what happens during playback. The final result will be a flat response. This situation is similar to what takes place when recording and playing discs. We boost treble during the recording of discs, but we play them back with the reverse of that boost. Surface noise is reduced in this process, but we also make the overall sound flat again.

The Dolby system is not that simple in action; it is not a simple cut and boost system. The amount of treble boost in recording varies with the dynamics of the program material, thereby keeping the tape from saturating at high recording levels. Thus, the playback will also have to vary in treble response in accordance with the dynamics of the program, but in the opposite direction from that of the recording process. Below some specific value of signal level, the Dolby system does not produce further treble boost. Above a certain signal level, the Dolby system does not act at all; the signal is flat. These limits must dovetail in the record/playback cycle or alterations of response will occur.

This is the reason why calibration of the system is so important.

Microphone Distortion

Q. I have two problems with distortion produced by the new, medium-priced electret-condenser mikes.

I have found that when subjected to very loud sounds that they produce intolerable distortion. An example of this is found in a recording we made of a large and loud stage band. Two takes were made. One was made using four condenser microphones run through a mixer, and then into a tape recorder. The other take was made with two dynamic microphones and the tape recorder. The condensers distorted on loud sounds, especially at peak levels. In the bass region, distortion was so high as to make the music virtually unrecognizable. The dynamic mikes did not know apparent distortion at any sound level, but lacked the beautiful, flat reproduction of the condenser microphones. When recording choirs and some orchestras, the condenser mikes caused pinning of the VU meters when sopranos decided to hit high notes.

Is there anything to be done about these two problems?

-James D. Caldwell, Jr., Odessa, Texas

A. I wonder whether the distortion you hear is actually created within the condenser microphones. Often, what happens is that the output signal from the mikes is so great that it overloads the early stages of a recorder or mixer.

These stages are ahead of any volume control, so that no improvement can be gained by reducing level. One sign of this condition is that the volume controls are down to perhaps 9 o'clock or less in order to obtain proper level.

There are attenuators which can be introduced between the mike and the input of a mixer or recorder. They are little more than voltage dividers. If you cannot locate one which has the appropriate connectors, you can make one up to suit your needs, with sufficient attenuation to reduce the signal to more manageable proportions. A 20-dB loss is usually sufficient.

Should it happen that you really do have distortion from the mikes themselves by virtue of an overloaded preamplifier, there is nothing you can do but to use mikes which will handle the sound levels you expect to encounter during recording.

It may be that the dynamic mikes do not produce as much signal as the condenser mikes do. This would mean, of course, that they would have less tendency to create the overload situation within the mixer or recorder. I noted with some interest, however, that in the take involving the use of the dynamic microphones, that you did not use the mixer. Thus, it may well be that the mixer is more susceptible to overload than is the tape recorder. To check this, why not make a recording using two condenser microphones feeding into the input of the tape recorder without using the mixer? If overload still takes place regardless of the setting of the volume controls on the tape recorder, you will know that either the early stages of the tape recorder are indeed overloaded or that the preamplifiers in the microphones themselves are being overloaded. While this latter possibility can occur, I have not seen it very often.

The fact that your meters sometimes "pin" when sopranos sing into the condenser mikes does not have much to do with the fact that you are using such mikes or that you are in a particular concert hall. It simply means that this is a characteristic of what happens when trying to record sopranos, especially when they are close to the microphones.

(Audio magazine, Herman Burstein)

= = = =