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Atmos, 7.1, 5. and Digital Signal to Noise

Pinewood emailed last week and asked if I would visit at the same time as the Dolby engineer and set up Theatre 6 for four standards:-  Atmos Performance, Atmos Home, 7.1 and 5.1

The front speakers were not a problem but I was concerned that the different level settings for the surrounds would reduce the digital signal to noise. The problem was caused by the gain range required to configure the surround  speakers for the different formats and speaker combinations.

The Formats

Theatre 6 has two left, two right, two left rear and two right rear surround speakers
Atmos performance uses all the surround speakers as individual point sources.
Atmos Home uses only one left, one right, one left back and one right back as individual point sources.
7.1 uses the surround speakers in pairs
5.1 combines the left side with left rear and combines Right side with right rear to make four left surround and four right surround speakers.

Setup Procedure:

1) Set up the room for Atmos performance and Atmos Home by adjusting the individual +/- 4dB trims provided by the TMC-1.
2) Set up the room for 7.1 using the global 7.1 surround trim in the TMC-1 to reduce the surround level
3) Set up the room for 5.1 using the global 5.1 surround trim in the TMC-1 to reduce the surround level.

The problem:

After calibration there eas approximately 10dB of attenuation on the surrounds for the 5.1 setup. 10dB, almost 2 bits of resolution lost, but this is not the only loss, consider the following.

1) The levels are set for 85dB SPL
2) The RMU has an insertion loss  of 10dB to allow for digital equalisation
3) Set to 5.1, on the surround channels there is a further 10dB insertion loss
4) Reduce the monitoring level to the Dolby Home level of 79dB SPL which adds a further 6dB insertion loss
5) Speaker trim range is  (+/-4dB) in the worst case there can be a further 4dB insertion loss.

The worst case insertion loss is 30dB or 5 bits, this would reduce a 16 bit signal to 11 bits. This is the case with the surround speakers in pairs, consider the possible insertion loss in a bigger studio where the surround speakers are in sets of 3, 4 or more.

The Solution:

The 10dB insertion loss in the Dolby unit is unavoidable and is required for any digital equaliser,  we have to accept this. (I would have limited the boost to a maximum of 6dB and reduced the insertion loss to 6dB but this is a design choice).

The DAD AX32/NTP Penta 720 also provides analogue gain control. I tested this for the gain control but the four 6dB relay steps cause unacceptable glitches in the audio. When used for  the speaker trims, Calibration Level adjust and surround trim the relay switching is no longer a problem!

When setup correctly the maximum extra digital insertion loss is 6dB so that when combined with the RMU gives a maximum 16dB insertion loss in any condition over the required range, a worthwhile improvement.

Update: originally I referred to "digital resolution" Florian Camerer pointed out that reducing the bit count does not reduce the resolution but reduces the Signal to Noise or dynamic range.