Bob's Blog #2: 7330 Internal Audio Levels

In Appendix B (Installation), the 7330 User Manual recommends adjusting the three receive audio pots for 1 V peak-to-peak as measured by an oscilloscope at test points TP9 (RX1), TP10 (RX2), and TP11 (RX3).

That specific level is maintained throughout the controller’s audio section. This blog explains why it was chosen.

7330 Audio Path, see Installation chapter page B-7

Unlike the ICs of years ago, many of today’s mixed-signal (analog/digital) ICs operate from a single, low-voltage (+3.3 V or +5 V) power supply. This allows the audio and digital sections of a product to use a common supply and simplifies the interface between them.

In the 7330, some mixed-signal ICs operate at +3.3 V (digital pots, DACs and DTMF decoders) and others at +5 V (audio switches and audio delays). The op amps operate at +5 V. In all cases, the audio signal rides on a DC bias voltage positioned between the positive supply and ground.

Given this description, it seems the 7330 should easily handle several volts of audio internally. One volt seems needlessly restrictive.

But as it turns out, we really DO need a generous amount of headroom. It’s easy to crank up receive audio levels when testing with sine wave tones and forget that voice audio is very “peaky” by nature. Excessively high levels result in distortion if the voice peaks are clipped at the power and ground rails. Clipping becomes even more severe when we add audio from a tone or speech generator, link receiver, etc.

On the other hand, we don’t want low or weak audio. The level needs to be much higher than any digital circuit noise to maintain a good signal/noise ratio, although that’s not much of a problem in the 7330. Circuit noise was minimized by using a four-layer PC board with power and ground planes, and a careful layout that separated the analog and digital sections and placed the power supply in between.

Considering all the above, 1 V p-p was chosen as the nominal level, and the other circuits were designed to match it.

For example, DTMF decoders have an optimum detection range. Insufficient audio leads to missed characters, and excessive audio leads to falsing, the detection of erroneous characters in the presence of voice or noise. The level fed to the decoder is correct when the internal audio level is 1 V p-p.

The on-board tone and stored speech circuits were designed to match an internal level of 1 V p-p.

By setting all three receive levels the same, listeners won’t hear a shift in audio level when different receivers are switched to a given transmitter.

So how do we match this internal level to our receivers and transmitters?

The 7330’s audio inputs and outputs were designed to handle a wide range of levels.

Each receive audio input has a level pot, a flat/de-emphasis jumper, and a normal/high gain jumper. (The high-gain position is for receivers with very low output.)

Each transmit audio output has a level pot and a normal/low level jumper. (The low-level position is used when driving a transmitter mic input.)

After first setting the internal level, the transmit output pot is set for the required level. Because transmit audio is leaving the controller, its level does not influence the 7330’s internal audio circuits.

The best instrument to use for setting levels is an oscilloscope because it displays peak-to-peak amplitudes. Setting up the controller by ear or with a DMM may not provide good results because there are many possible pot setting combinations that appear to work, but don’t.

For example, let’s assume the receive level is set very low and the transmit level very high to compensate. The repeat audio may sound fine, but if the 7330’s internal level is too low, we may encounter missed DTMF digits and the need to turn down the CW and stored speech levels considerably to match the repeat audio.

At the other extreme, assume the receive level is set very high and the transmit level very low to compensate. The repeat audio may sound fine, but if the 7330’s internal level is too high, we may find occasional muting of repeat audio (mistaken for DTMF) and the need to turn up the CW and stored speech levels considerably.

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