Audio - Unity System Gain

[EA7KIR]

I have an HDMI Audio Extractor, which I can use to calibrate an analogue PPM to read PPM 4 BBC/EBU lineup level, by having a Mac (or PC) generate a 400Hz sine wave tone at -18dB FS and sending it out over HDMI. I also know the Mac's volume control does not affect the level going out, so it's reasonable to assume the HDMI digital signal is carrying the same level.

If I didn't have a PPM, I could have used an oscilloscope, or even a voltage test meter, and noted the voltage reading. All I need is a reference.

DATV is, err, digital, so when I receive back my own transmission I should expect it to arrive back at the same level. But when I connect the receiver's HDMI output to the HDMI Audio Extractor it isn't

The return level is 3dB higher than expected. The system does not have unity gain.

I don't have the test gear to inspect a digital bit stream, but I can reduce the volume setting of the decoder - ffplay in my case, but VLC is the same. Reducing its volume by 3dB will make my PPM read 4, but does this solve the problem?

Well it certainly makes the system appear to have unity gain, but what about the gain distribution - or system headroom if you prefer.

It turns out that if I have the MAC/PC output 0dB FS (ie maximum before clip), and verify that it doesn't sound distorted, the received audio becomes distorted. Slowing backing off the send level reduces distortion to the point where it completely disappear when sending exactly -3dB FS.

So where is this 3dB gain being introduced and how can I fix it? Any ideas?