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Author Topic: Gain, bandwidth, output levels wtf??  (Read 668 times)

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x838nwy

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Gain, bandwidth, output levels wtf??
« on: July 16, 2015, 07:01:31 AM »
Background
In a Facebook group about audio stuff, someone posted saying that they prefer another DAC to a mytek 128DSD despite the mytek getting rave reviews on Jude-Fi. I was among a few people who seconded the opinion that I found the mytek to be shite and artificial-sounding, forced for lack of a better word. This is okay, until...
A fairly prominent reviewer mentioned that there're jumpers inside the mytek which allows the gain the be reduced (by -6dB?) in case it causes clipping with some pre-amps. Again, this was okay, until he went on to explain that:

WTF part I (translated)
Quote (selected)
"The designer [of the Mytek] designed the output level to be very high, specifically for DSD playback. This is because decoding DSD without digital volume compensation results in a very quite signal. Many times lower than the average gain at the output from decoding PCM"

I asked whether this would make PCM tracks crazy loud and guessed that the gain switch was probably more for connecting to a power amp directly (as the mytek also acts as a pre amplifier). He then went on to explain that:

WTF part II (translated)
Quote (selected)
"Mytek designed the DAC to be able to decode native DSD which decodes into a high bandwidth analog signal. If one chooses a 70kHz signal one would get an analog output signal ranging from 0-70kHz which reduces the average level of the analog signal. The designer therefore chose a high gain for the signal from the DAC chip to match the output level when decoding PCM..."

Baffled with the whole 70kHz thing, I asked how the hell the band bandwidth and gain are related, to which I was told:

WTF part III (translated)
Quote (selected)
"If an (single stage) amp designer wants his amplifier to have a wide bandwidth, the amplifier must have reduced gain. If she/he wants to have a higher gain, the bandwidth must be reduced. "

Baffled as I am, this has made me question my (limited) understanding of stuff... and as such I will be forever grateful if the learned members of this board may enlighten me:

- Does what this guy is saying make any sense?
- Are the gain and bandwidth of an amplifier related in this manner?
- I think there's some compensation going from DSD->PCM in terms of volume but is that not on a software level?
- Why would Mytek choose a weird output voltage level?
- Could the apparent cranes of the mytek be related to its wild output level? I have no truck with Mytek, but I for one didn't even think to look at its output level so may be I (mis)judged it based on the wrong settings?
- A long time ago, I worked with something called the gain-bandwidth product which iirc is something like an indicator of the energy of a signal. How's that related to the amp's design and gain settings?

Thank you

C
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ultrabike

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Re: Gain, bandwidth, output levels wtf??
« Reply #1 on: July 16, 2015, 08:58:13 AM »
While a Delta-Sigma might require some gain in the feedback, I'm not sure why the output level needs to be high.

I suspect there might be some jedi mind tricks going on there.
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takato14

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Re: Gain, bandwidth, output levels wtf??
« Reply #2 on: July 16, 2015, 01:59:53 PM »
I am almost 100% positive this guy is full of shit
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This industry is really fucking broken

Chris F

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Re: Gain, bandwidth, output levels wtf??
« Reply #3 on: July 16, 2015, 04:17:54 PM »
I need to look up the exact spec but I'm almost certain DSD is normally 6dB down compared to the equivalent PCM so that part is not BS.  If you look in the software options for DSD->PCM conversions there is nearly always an option for +6dB gain compensation because of this.

Edit - Found it in the Weiss Saracon manual:
Quote (selected)
3.3.4 Gain during D2P
The gain during D2P conversion defaults to +6dB
because the SACD/DSD encoding’s maximal amplitude
corresponds to -6dB in the PCM domain
(those -6dB are usually referred to as 0dB SACD,
see section C of the appendix).
Note 1 It must be payed attention though
when reinflating the amplitude back to 0dB
PCM because the DSD signal can contain
amplitudes greater than those -6dB PCM

Quote (selected)
C.2 Zero dB Audio Reference Level
(D.2)
The SACD Zero dB Audio Reference Level, referred
to as 0dB SACD, corresponds to a sine wave
with a peak amplitude equal to 50% of the theoretical
maximum DSD signal level.
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ultrabike

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Re: Gain, bandwidth, output levels wtf??
« Reply #4 on: July 16, 2015, 04:23:17 PM »
From what I've been reading in the interwebs, the -6dB is per standard... and it is not always respected.
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x838nwy

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Re: Gain, bandwidth, output levels wtf??
« Reply #5 on: July 16, 2015, 05:35:18 PM »
The whole 6dB thing is on the software or chip level, right? I mean the gain on ther output stage has nothing to do with that cos it doesn't change between fole types.

Plus what's this about larger amp bandwidth = less gain???
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Marvey

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Re: Gain, bandwidth, output levels wtf??
« Reply #6 on: July 16, 2015, 05:42:00 PM »
  • Depending on the output circuit, lower gain = higher feedback, which does tend to increase bandwidth. Hard to say if this is even necessary or relevant. Pro stuff does tend to have hot outputs.
  • The bandwidth argument is nonsense in any case. DSD must be filtered for ultrasonics because of noise shaping for the 1-bit processing (humans don't hear in binary, and simulating analog with 1-bit switching takes some work). The thing behind DSD is that noise rapidly rises after 24khz and peaks around 50kHz. This needs to be filtered in the analog domain with either a steep slope filter at higher frequency or a shallower slope filter at lower frequency. Either way, DSD output won't have any information by 70kHz. DSD bandwidth is about equivalent to 48kHz PCM, but with higher bit depth, like up to 28-30 bits (or something like that) in the sweet spot of the audio range.
CONCLUSION: that guy is full of poo  People come up with all sorts of lame excuses or bonehead explanations to vociferously defend stuff that some authority told them to waste their money on.
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Armaegis

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Re: Gain, bandwidth, output levels wtf??
« Reply #7 on: July 16, 2015, 06:58:45 PM »
Sounds like the guy read an opamp spec sheet and got to the one paragraph that explains gain vs bandwidth and drew all his conclusions from there.
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Marvey

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Re: Gain, bandwidth, output levels wtf??
« Reply #8 on: July 16, 2015, 07:01:54 PM »
Fricking Sony and DSD. All they did was cause customer confusion with no increase in sound quality for 99.9% of the music out there.
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