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In an analog broadcast audio chain, an automatic gain control (AGC) or compressor is employed to maintain a nearly constant average audio level fed to the over-the-air transmitter.

In a digital audio chain, say as in a streaming radio station, what is the equivalent technique employed to maintain a nearly-constant average audio level? OR, is there no need to?

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  • \$\begingroup\$ Use a digital compressor. \$\endgroup\$ – Andy aka Apr 11 at 8:36
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When broadcasting over the air, whether the audio chain is analog, digital, or a hybrid, it's critical not to exceed the peak modulation level that the transmitter (AM or FM) can handle. That's called overmodulation, and it not only distorts the audio, it also can cause spurious signals outside the legal bandwidth assigned to the station.

Preventing that is technically the job of a limiter, which only reduces the signal level when it exceeds that 100% modulation limit. It's a little different from a compressor which reduces excessive peaks, but may also be set to boost the signal when it's too low.

You don't necessarily want a "nearly constant average audio level" if you want to faithfully transmit classical music. On the other hand, if you are doing voice interviews and passing microphones around in a noisy environment, you may want plenty of compression to keep the speech levels fairly constant.

The overmodulation/spurious signals issue doesn't apply to digital streaming, but you still should avoid clipping and distortion, and some combination of compression and/or limiting will be helpful in processing your audio. Most audio consoles or audio software will offer a variety of these options.

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I think you're mixing two things:

  • The RF power being transmitted to the antenna

  • The audio signal peak level

In analog AM transmission they're related since audio signal modulates the amplitude of the RF carrier. So if you want to use the full power of your transmitter, but not exceed it, then you need automatic gain control to set the analog signal peak level to where you want it.

But in analog FM and digital transmission they are not, the transmitted RF power is pretty much constant. In FM the frequency changes but the amplitude stays constant, and in digital transmission it depends on the modulation scheme used, but most will try to maximize signal to noise ratio by keeping RF transmitted power high and constant on average, for example by modulating phase. If it also uses amplitude modulation, the signal is scrambled to keep average power constant and not dependent on data being transmitted, which gives maximum SNR.

In both latter cases you still need to maximize the use of available audio signal amplitude range without clipping, so you'd still use some form of AGC, but it would rather be called "limiter/compressor" or something like that. It can be implemented as an analog effects box in your rack, or as software.

For example compression reduces the dynamic range, to make soft parts of the music a bit louder, since the signal to noise ratio of AM/FM is not that good and people listen in their car where there is a lot of noise anyway.

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