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The bit game

Paul's Posts — 14 August 2011

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The bit game

We read about digital audio devices boasting bit depths up to 32 bits and understand more is better.  But better at what?

Bit depth is a measure of the signal to noise ratio (how much louder the music can be than the background noise) and also tells us dynamic range (the softest to the loudest levels possible).  The measurement used is dB (decibels) and just to give you a reference, every 6dB is twice as loud as the last 6dB.  A 16 bit system has 96dB of range, a 24 bit system 144dB.

I am going to give you one more figure and then onto the point of the article: the human ear can discern (on a good day) 124dB which is a mere 21 bits.  So why do we care about higher bit rates like 32 and beyond?  Does it really matter?

The answer is, as usual, not so simple.  On the one hand I would tell you that right now, with 99.99999% of recordings and audio equipment out there, no.  Could it someday matter?  Yes.

Here’s the deal.  As DAC designers have pushed for more bits to extend the dynamics and increase the S/N ratio, recording engineers and amp/preamp designers are still living in the past because their maximum loudness levels are identical and fixed.  What’s actually improved is the other side of the recording: how quiet it can get.

Think of it like this.  Imagine you’re in a noisy cafe listening to a musician playing his guitar in the corner of the cafe.  When he sings loudly you hear him rise above the noise of the crowd and that sets the maximum level he can sing at.  Now imagine the crowd leaving and the background noise dropping to a very quiet level.  The singer still hits the same loud notes but now you can hear all the small nuances you missed because of the background noise.  That’s what happens when we go from 16 bits to 24 bits.

In this analogy you’ve gained the ability to hear many small nuances from the singer – but not all.  You can’t, for example, hear his heartbeat or even the slight movements he makes in his seat.  These are just too quiet to hear – but in fact – going from 24 bits to 32 bits buys you enough quietness to where you should be able to hear them – but that extra quietness is wasted because you can’t hear it even if it was on the recording (which it is not).

If, on the other hand, system engineers and recordists went the other way – adding dB’s of maximum loudness instead of quietness – then when a waiter in the cafe knocked over a pile of plates, the dynamic range you got would be so realistic and perceivable that you’d be startled out of your chair; the sound so much louder than the singer.

Technically, we already have the dynamic range needed, we’re just not taking advantage of it and it is wasted.

Tomorrow we’ll examine why vinyl, which has even less dynamic range than a 16 bit CD, seems to have greater dynamics.

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About Author

Paul McGowan

Paul McGowan is the CEO and co-founder of PS Audio Inc. a Boulder Colorado design and manufacturing company of high-end audio products and services. McGowan has been designing and building high-end products for nearly 40 years. Hobbies include skiing, music, hiking, artisan bread baking, kick boxing and cooking. He lives in Boulder Colorado with his wife Terri and his 4 sons.

(3) Readers Comments

  1. Frode Roed

    August 14, 2011 at 7:17 am

    Very interesting.
    I think what is mentioned in this paper should also be be considered mentioned in this respect:
    http://www.wadia.com/technology/technicalpapers/Digital_Volume_Control_2.pdf

    Frode :)

  2. August 14, 2011 at 7:37 am

    Thanks Frode. The Wadia paper shows how a 16 bit signal is upsampled to 22 bits and then uses the extra 6 bits to shift the signal so you can attenuate without losing the original. This is the same thing we do in the PWD, although we actually upsample to 24 bits and then throw away the last 8 leaving the original 16 untouched – which is why we suggest that when folks use the PWD volume control, you get a free ride above 50 on the volume control, but then start losing resolution below that.

    One of the problems with this approach comes when we play higher resolution audio streams like 24 bits to start with. Now we can’t upsample anymore and when you use the volume control you really are throwing away bits – but as mentioned in this article, in many recording (most actually) it’s questionable whether that matters or not.

    Thanks for posting!

  3. Pingback: Vinyl Vs. CD | PSTracks

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