TV channels (terrestrial DTV, satellite and, I guess, cable) may be broadcast in one of three formats:

PCM - which may or may not be Dolby Surround encoded;
DD2.0 - which is stereo Dolby Digital and usually Dolby Surround encoded;
DD5.1 - which is full fat discrete multi-channel (but which may actually contain anything from 1.0 to 5.1).

Typically, SD channels use PCM or DD5.1 while HD channels use DD2.0 and DD5.1

Different broadcasters (*) use different bit-rates for DD (2.0 and 5.1) which may have implications for sound quality. They may also send different dialogue normalisation meta-data which affects the DD decoding (I think for 5.1 only) and would be see on an M processor as a brief display something like "DialNorm -4" when the DD5.1 signal is detected (at start or after a pause).

On that technical platform, the broadcaster then vary the volume level on a content-by-content basis. So, for example, E4HD broadcasts TBBT in DD5.1 but cuts to DD2.0 for adverts, however those adverts are blasted out a stupidly high level. CH5HD seem to broadcast everything in DD5.1 but apply terrible compression which destroys the impact of car-crashes, etc. but the adverts are not too bad.

Depending on the channel then it is possible for SD (PCM) to appear loud and HD(DDx.x) to appear quiet, and vice-versa.

Another variation is that I find downloads from SkyHD to need +6 -> +8 on the volume compared with the broadcast or recorded-from-transmission versions of the same content.

The 518 option can solve (+ or -) the PCM levelling to some extent to align it with DD but cannot correct the volume of DD itself.

I don't think any of this is something that the HD621 can solve, nor even is it designed to solve it as it passes the undecoded DD signal through for decoding in the processor.

The M processor will apply your chosen synthetic surround mode to PCM and DD2.0 but note that you must set that default up separately for each of the two formats. DD5.1 (usually) invokes Cinema, to which you can apply compression for, for example, late-night listening.



(*) I'm limiting this overview to UK broadcasters

In the spirit of Spinaltap, I would like to confirm that I am not involved in the corrupt theft of a global sport.