Tips on analog tape, tape recorders, tape bias, logic inputs, and logic control

November 1, 2007

Analog Tape

Part 2, Electronics

Figure 1. The standardized tape recording and playback EQ curves.

The next aspect of analog tape recording is an interesting one, for it can also apply to audio transmission on long circuits (telephone lines), to FM radio reception and to TV signals. The magnetic recording head is an inductive device and its impedance varies directly with the frequency of the sound being recorded. In addition, the magnetic field available to record on the tape is directly related to the current flowing through the head. Because of this, the high frequencies are not recorded as easily as the lower ones. This is corrected by adding resonant circuits to the audio input stages in front of the record head, and to the playback stages that follow the playback head. In tape recording and telco terms this is referred to as equalization. In FM radio it is pre-emphasis/de-emphasis.

The record equalization circuitry boosts the weak parts of the spectrum, so the tape can "see" and record them. To bring things back to the proper relationship, a similar reduction in those frequencies is introduced in the playback circuit: Reproduce equalization. Often there are separate EQ adjustments for the high frequencies (HF) and the low frequencies (LF). Better quality recorders have more stages and sophisticated EQ circuitry. Lesser quality machines may only have one adjustment, or none at all.

The tape's sensitivity to magnetic fields at different wavelengths is also not uniform. To compensate for this, a super-audible signal (typically about 100kHz) is imposed on recorded signal, usually at a very high level. This signal is called bias. Similar to bias in an amplifier, this signal is mixed with the recorded audio, and it linearizes the record function at high frequencies. Proper adjustment of the level of this signal reduces distortion and noise.

To assure consistently high quality recording, only one specific brand and type of tape should be used at a studio. Each machine should give the same results once all of the adjustments are set up for that house tape.

In a multi-track recorder, all of the adjustments (HF and LF record, reproduce EQ and bias) must be done separately for each track. This requires a lot of set-up time, and is just one of the reasons for the digital migration to systems such as Adobe Audition and Pro Tools.

It's simple logic

Remote starts from console are just the beginning. A simple pulse is the most common form of control — from lighting the on-air light to firing the commercial break from the automation. And over the years, so many different devices with different hookups have been introduced, which means every time a new piece of equipment comes into the rack room, the fun begins again.

Figure 2. A pull-up resistor can supply the needed voltage for the logic pulse.

Most logic inputs have a voltage reference, and usually the equipment supplies it. When this voltage changes (by being connected to the common terminal) the equipment does its thing. Some devices, such as a popular ISDN codec, requires the user to provide a reference voltage to the inputs. An external power supply can be used, or a pull-up resistor tied to the voltage pin on the logic connector can supply the voltage. Figure 2 shows the connector with a 330 ohm pull-up resistor pulling the high side of the four inputs up. A switch between the low side of each input and the common terminal will change the state and send that change down the line.

Output circuitry can also require a voltage source. Another popular ISDN/Ethernet codec requires voltage to receive the output (and this includes the outputs you can use to tell when it is in use).

It gets a little confusing when a logic pulse has to be used to trigger two different pieces of equipment at the same time. For example, a satellite receiver's closures must be sent to the studio on a T1 link as well as a back-up ISDN. If both devices provide a voltage, the voltages must be isolated. This can be done by inserting two diodes.

If more logic pulses need to be sent to many devices, a better way is to use a pulse distributor, such as those offered by Henry Engineering, Broadcast Tools and Radio Design Labs.

Landry is an audio maintenance engineer at CBS Radio/Westwood One, New York.

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