Geoff and others - truncation of bit depth and poor low volume audio performance

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seethroughyou Posted: Fri, Jan 15 2016 12:58 PM

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Several years back before I sold my Beolab 5s (using SPDIF only) I lent them to my father-in-law for a year whilst we moved out and had the house refurbished. In his living room sat a Beosound 5-to- spidf connection-to-Beolab 5 using ripped CD quality music on the Beomaster 5.

Side by side, sat his more modest hifi = a pair of Heybrook speakers from Richserounds, Marantz CD player and Roksan Kandy integrated amp.

The Beolab 5 was as expected far superior in every way in terms of sound quality at middling to high volume levels but I noted an interesting phenomenon that I really wasn't expecting;  his modest system sounded more detailed and superior at low volume. After a few days, I preferred listening to his system rather than mine in the evenings when we needed to keep the volume low.

Unlike analogue preamplifiers which reduce both the source signal and the noise floor at the same time, I have heard that digital volume controls in DACs reduce only the source signal not the noise and one can find the sound loses detail (the quiet instruments in piece become inaudible) and quality drops as you turn the volume down unless some special dithering techniques are applied and the DAC uses far more bits for this volume control operation than the source format - i.e. have a 35 bit DAC doing volume control on 16 bit or 24 bit files.

There may of course be other explanations for this poor low volume performance compared to his modest system that I haven't thought of...perhaps the drivers in the Heybrook are more sensitive...

1. What are explanations could there be?

2. How does B&O do volume control in the digital era and what precautions does it take to prevent this phenomena?

3. Have other's who have a second system that's not B&O, noticed that DAC-to-amp-to- speakers sounds poor compared to source-to-preamp-amp-to-speakers?

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Jeff replied on Fri, Jan 15 2016 1:33 PM

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Was the loudness contour switched on on his cheaper stereo?

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riverstyx replied on Fri, Jan 15 2016 2:19 PM

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If my memory serves me correctly (I'm not able to check right now), the block diagram in the BL5 service manual indicates the volume control is performed on the analogue signal after the processing and ADC stages (just before the signal is fed to the amplifiers). So for this product there should be no reduction on bit depth at low volume levels.

You are correct thought that this can be an issue where the level of a digital signal is controlled at an earlier stage, eg. where a source has a volume control which reduces the level before it is output digitally (say via SPDIF).

The human ear is not linear in its perception of different frequencies, and becomes much less so as the volume level decreases (this was what the 'loudness' filter was originally designed to compensate for, although it is often mis-used).

The audible differences between one system and another and in particular a listeners preference for one over another can often be influenced by even very small differences in volume level and it may be that you just have a preference for a particular tone or colouration of the output - this difference could be a result of the volume level, the EQ applied, the speakers, the room (and any room compensation), or more likely a combination of all these factors.

Kind regards,

Martin.

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vikinger replied on Fri, Jan 15 2016 2:38 PM

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For some time I played from my Olive One via my 1970's BM2000 into MK1 S45's. Then I decided to try connecting the Olive with its internal Class D amplifiers direct to the S45's. I was astonished to find that there was absolutely no perceptible difference in sound quality, and at low volumes the Olive played the same as the did BM2000 with the loudness button down.

When using the Olive connected to the BM2000 I had the Olive on max volume and reduced the output using the BM2000 volume control. Obviously when using the Olive direct the volume is reduced digitally and is theoretically of poorer quality but I can't tell the difference.

Graham

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seethroughyou replied on Fri, Jan 15 2016 4:24 PM

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Jeff:

Was the loudness contour switched on on his cheaper stereo?

Good point Jeff, no loudness button on his amp to activate just source selection.