# Is there a way to get FreeDV output level to follow the input audio amplitude?

I'm using a commonly available imbedded firmware implementation of FreeDV with an SSB HF transceiver.

It seems that FreeDV encodes the input audio signal in both the frequency and time domains, which means that the output signal amplitude doesn't follow the input signal amplitude, but rather the output level from FreeDV is always high, even with no input audio.

This is a real problem for some SSB transmitters and amplifiers which are designed to not operate at constant 100 % output, and the continuous high output can cause them to overheat.

Also the continuous 100 % output causes batteries in portable SSB equipment to go flat much quicker.

In addition, an SSB signal that is always close to 100 % modulation level doesn't help with conservation of bandwidth or with reducing interference to other operators.

To me it seems that FreeDV results in wasted energy when used with SSB. I'm surprised that FreeDV would operate this way, especially when it's advertised for SSB usage, which makes me think i'm missing something.

Is there any way to make FreeDV encode the input audio signal only in the frequency domain so that the input signal level is preserved ie: no audio input produces no output from FreeDV ?

It seems that FreeDV encodes the input audio signal in both the frequency and time domains, which means that the output signal amplitude doesn't follow the input signal amplitude, but rather the output level from FreeDV is always high, even with no input audio.

Nah, that's not really the case. FreeDV uses a multicarrier scheme with 14 orthogonal carriers. Each of these carriers a (differential) PSK signal – i.e. one with constant envelope. Math (to be specific, the Central Limit Theorem) says that if you add up "many" independent, but identically distributed random variables (combining the 14 carriers into one signal), you end up with a (approximately) Gaussian distribution.

And that is good. It means we get the most information across, given a fixed output power. Any other amplitude distribution is worse (for the continuous-amplitude case, at least; see: Differential Entropy).

More info with same amplitude means we have, within the same bandwidth, more "room" to make our transmission robust. That's one of the reasons why FreeDV under low-SNR scenarios is usually better than analog SSB.

This is a real problem for some SSB transmitters and amplifiers which are designed to not operate at constant 100 % output, and the continuous high output can cause them to overheat.

These things will overheat because of the power put through them. So, we can find a power that they can continuous work with, and just work with that.

In other words: