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DAC that handles intersample overs
post Oct 23 2012, 16:32
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I was tipped off to this on an audio mailing list and thought it might be of interest here -- Benchmark has released a consumer DAC that (among other features) allows 3.5dB of headroom to account for correct reconstruction of intersample overs (peaks between samples that exceed 0dbFS, which are said to be "common in commercial releases")


High Headroom DSP - with 3.5 dB "Excess" Digital Headroom
All of the digital processing in the DAC2 HGC is designed to handle signals as high as +3.5 dBFS. Most digital systems clip signals that exceed 0 dBFS. The 0 dBFS limitation seems reasonable, as 0 dBFS is the highest sinusoidal signal level that can be represented in a digital system. However, a detailed investigation of the mathematics of PCM digital systems will reveal that inter-sample peaks may reach levels slightly higher than +3 dBFS while individual samples never exceed 0 dBFS. These inter-sample overs are common in commercial releases, and are of no consequence in a PCM system until they reach an interpolation process. But, for a variety of reasons, virtually all audio D/A converters use an interpolation process. The interpolation process is absolutely necessary to achieve 24-bit state-of-the art conversion performance. Unfortunately, inter-sample overs cause clipping in most interpolators. This clipping produces distortion products that are non-harmonic and non-musical . We believe these broadband distortion products often add a harshness or false high-frequency sparkle to digital reproduction. The DAC2 HGC avoids these problems by maintaining at least 3.5 dB of headroom in the entire conversion system. We believe this added headroom is a groundbreaking improvement.

disclaimer: i neither work for Benchmark nor own a Benchmark DAC ;> Claims about reduced 'high frequency sparkle' taken with a grain of salt.

This post has been edited by krabapple: Oct 23 2012, 16:33
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post Oct 24 2012, 13:13
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I know the OP is talking about inter sample overs specifically but I am both intrigued and puzzed by what is going on with modern pop and electronic releases as regards digital clipping.

I have done some experiments and would welcome comments and suggestions if anyone else is interested.

I downloaded a brand new commercial MP3 release via Beatport, the popular on-line music store. Loaded it into Traktor, effectively a professional grade media player. All controls left on unity gain (no effects, EQ etc) and played it via ASIO and WASAPI drivers. So called 'bit perfect' playback.

The output meter in Trakor indicated regular red overs. Practically every beat. I couldn't actually hear clipping unless I applied more EQ. However Trakor docs state that a limiter is always applied to the OP. So I wasn't really outputting an over scale signal. What the meter was telling me was that I was losing dynamic range and hence 'punch' from my performance.

So I looked at the metering on my DAC (MOTU Ultralite) using it's in built facilities (CueMix). Both input and output meters indicated frequent red overs. Unchecking the limiter in Traktor had little or no effect. Cuemix said I was clipping. Although it wasn't audible, at least to my ears.

So I loaded the track into Audacity and analyzed for clipping. Sure enough the waveform was a sea of red. Both amplify and normalize suggested I needed to take off a whopping 0.9dB to stay within scale. Which I did. Whereupon Audacity reported no errors on playback. The waveform looked like a regulation crew cut haircut. When I exported the normalized file and loaded back into Traktor the playback now showed no clipping. Cuemix on the other hand still insisted the file was clipping at the DAC. I had to return to Audacity and take off another 1dB (i.e. -1.9dB) before the track played 'clean'.

I carried out a frequency analysis on the original track in Audacity. The only place it came closer than -24dB was very low down (>50hz) where it actually touches 0.

I have absolutely no idea what is going on here. perhaps the meters are inaccurate, perhaps they deliberately over read as a safety precaution, perhaps popular modern music is either deliberately or incompetently released mildly clipped in a quest for volume on the basis that most users will not hear it as it is so low down in the audio spectrum?

Whatever it certainly raises doubts on the commonly expressed audiophile recommendation to always play back domestic audio via 'bit perfect' ASIO or WASAPI drivers.

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Posts in this topic
- krabapple   DAC that handles intersample overs   Oct 23 2012, 16:32
- - greynol   QUOTE (krabapple @ Oct 23 2012, 08:32) We...   Oct 23 2012, 17:36
- - RonaldDumsfeld   I wonder how they do it? How can you represent a v...   Oct 23 2012, 19:20
|- - Arnold B. Krueger   QUOTE (RonaldDumsfeld @ Oct 23 2012, 14:2...   Oct 23 2012, 20:25
|- - Arnold B. Krueger   QUOTE (RonaldDumsfeld @ Oct 23 2012, 14:2...   Oct 26 2012, 15:12
|- - pbelkner   QUOTE (Arnold B. Krueger @ Oct 26 2012, 15...   Oct 27 2012, 05:56
- - DVDdoug   QUOTE I wonder how they do it? How can you represe...   Oct 23 2012, 20:15
- - AndyH-ha   The DAC's output is an analogue voltage swing....   Oct 23 2012, 22:07
|- - Arnold B. Krueger   QUOTE (AndyH-ha @ Oct 23 2012, 17:07...   Oct 24 2012, 13:51
- - [JAZ]   There are several sentences that sound strange to ...   Oct 23 2012, 22:17
- - Wombat   I am absolutely no expert at this but may it be as...   Oct 23 2012, 22:30
- - AndyH-ha   Considering the general state of decent hardware, ...   Oct 23 2012, 23:16
|- - Arnold B. Krueger   QUOTE (AndyH-ha @ Oct 23 2012, 18:16...   Oct 24 2012, 13:59
- - pdq   Upsampling would involove multiplying a set of dat...   Oct 24 2012, 10:55
- - RonaldDumsfeld   I know the OP is talking about inter sample overs ...   Oct 24 2012, 13:13
|- - pbelkner   QUOTE (RonaldDumsfeld @ Oct 24 2012, 13:1...   Oct 24 2012, 15:45
- - 2Bdecided   @RonaldDumsfeld, many modern pop CDs are near clip...   Oct 24 2012, 15:22
|- - greynol   QUOTE (2Bdecided @ Oct 24 2012, 07:22) Th...   Oct 24 2012, 15:32
|- - 2Bdecided   QUOTE (greynol @ Oct 24 2012, 15:32) QUOT...   Oct 24 2012, 16:46
- - lvqcl   Probably this - http://www.hydrogenaudio.org/foru...   Oct 24 2012, 17:00
- - Speedskater   A sometimes technical and sometimes not E-mail gro...   Oct 24 2012, 18:06
- - gymnos   some of the topics of this discussion were covered...   Oct 24 2012, 21:47
|- - krabapple   Yes, but that work is a decade old. Are pro DACs ...   Oct 25 2012, 05:37
- - RonaldDumsfeld   QUOTE Could you give r128gain a try? I'd love...   Oct 25 2012, 15:31
|- - pbelkner   QUOTE (RonaldDumsfeld @ Oct 25 2012, 15:3...   Oct 25 2012, 16:19
- - RonaldDumsfeld   ^^ I'd already done that. It opens a command ...   Oct 25 2012, 18:42
- - Wombat   I wonder if these inter-sample peaks are a problem...   Oct 27 2012, 16:55
|- - pbelkner   QUOTE (Wombat @ Oct 27 2012, 16:55) I won...   Oct 27 2012, 18:08
- - lvqcl   Several posts in this thread are relevant to this ...   Oct 27 2012, 19:22
- - bandpass   Sounds like this DAC is just what I need for my am...   Oct 27 2012, 19:45
- - Dynamic   Thomas Lund has measured intersample over distorti...   Oct 28 2012, 01:12

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