Bob's Blog #2: 7330 Internal Audio Levels

(Revised 07-03-2018)

The 7330 User Manual’s Appendix B (Installation) 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).

The 1 V p-p level is maintained throughout the audio section of the controller. This blog explains why that level was chosen.

7330 Audio Path, see Installation chapter page B-7

Audio ICs of years ago operated from ±12 V or ±15 V supplies. Today we use mixed-signal (that is, analog/digital) ICs that operate from a single supply of +5 V, +3.3 V, or even less. This allows the audio and digital sections of a product to use a common supply and simplifies the interface between them.

In the 7330, digital pots, digital-to-analog converters, and DTMF decoders operate at +3.3 V. Op amps, audio switches, and audio delay ICs operate at +5 V. In all cases, the audio signal rides on a DC bias voltage that’s midway 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 do want a generous amount of headroom. It may not seem necessary when we test with sine wave tones, but we don’t want to forget that voice audio is very “peaky” by nature. When voice peaks are clipped at the power and ground rails, distortion results. And the clipping becomes more severe when the repeat audio is summed with additional audio from the tone/speech generator, a link receiver, or other sources.

Admittedly, we don’t want our audio to be too low. The level needs to be much higher than any digital 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 audio circuits were designed around it. What’s involved with that?

Consider the DTMF decoder. It has 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. We designed the DTMF decoder circuit so that the level fed to the decoder is correct when the internal audio level is 1 V p-p.

Similarly, we designed the tone generators and the stored speech circuits to output audio that easily adjusts above and below a repeat audio level of 1 V p-p.

And by picking a value and asking you to set all the receive levels to that same value, listeners won’t hear a shift in audio level when different receivers are switched to a given transmitter.

So how do we match this 1 V p-p internal level to our receivers and transmitters? The 7330’s audio inputs and outputs were designed to handle a wide range of real-world 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 receive audio pots for an internal level of 1 V p-p, we set the transmit output pot for the best transmitter level. Because this audio is leaving the controller, the pot setting does not affect other audio circuits.

The best instrument to use for setting levels is an oscilloscope because it displays the correct peak-to-peak amplitudes. Setting up the controller by ear or with a DMM isn’t very accurate. Why do we need accuracy? Because there are many RX and TX pot setting combinations that appear to work, but don’t.

For example, let’s assume we’ve set the receive audio pots for an internal level that is too low and turned up the transmit pot to compensate at the transmitter. The repeat audio may sound fine, but if the 7330’s internal level is too low, we may find that the DTMF decoder misses digits. We may notice that we need to turn the tone and speech levels way down to match the repeat audio.

At the other extreme, assume we’ve set the receive audio pots for an internal level that is too high and reduced the transmit level to compensate. The repeat audio may sound fine, but we may find occasional muting of repeat audio when certain voices are mistaken for DTMF. We may notice that we need to turn the tone and speech levels way up to match the repeat audio.

One other thing: Confusion can occur when we talk about deviation.

Repeater controllers don’t deal with modulation or demodulation, so they don’t deal with deviation directly. They deal with audio levels in terms of volts peak-to-peak.

But we had put this instruction in the manual: “Feed the receiver with a fully-deviated 1 kHz sine wave tone from a service monitor.”

The problem is that repeater technicians use varying standards when setting system levels, and “fully deviated” means different things to different people.

If you are running an analog repeater in FM wideband (16K0F3E) like most of today’s amateur radio repeaters, fully deviated means about 4.5 kHz.

If you are running an analog repeater in FM narrowband (11K0F3E) like the new land mobile standard, fully deviated means about 2.5 kHz.

If you have the 7330 port connected to an Allstar audio interface, the simpleusb-tune-menu will let you set levels to match a deviation of 3 kHz or 5 kHz.

Many operators of FM wideband repeaters use a 1 kHz audio tone deviated at 3 kHz to set levels in the controller. What happens?

If we use 3 kHz deviation to set 1 V p-p as our internal level, the speech and tones at their default settings will deviate 3 kHz on transmit. The average user deviates between 4 kHz and 5 kHz maximum, so the default levels for the speech and tones end up slightly on the low side.

But if instead we use 5 kHz deviation to set 1 V p-p as our internal level, the speech and tones at their default settings will deviate 5 kHz on transmit. With an average user deviating between 4 kHz and 5 kHz max, the default levels for the speech and tones are louder than the users.

The amount of deviation you choose to use for an FM wideband repeater to set the internal audio level of the controller is mostly a matter of preference. We do not recommend going below 3 kHz deviation. For other types of interfaces, pick a level that makes sense for that interface.

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