Understanding sample rate changes with Decibel and DACs...

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Understanding sample rate changes with Decibel and DACs...

Post by mullet »

Ok... I'm still a bit confused about the sample rate adjustment settings and I wanted to make a separate thread instead of hijacking the other one about sample rates...

This is what was said in the last post:

"As I understand it this is what happens:
1. If you check the option, Decibel will adjust the DAC's transfer rate to send the file in the best quality possible (in most cases, the exact file resolution, in some edge cases, as close as possible with software resampling)
2. If you uncheck the option, Decibel will do up-sampling or down-sampling in software to send the file to the DAC in the sample rate that the DAC is originally set to."

So I'll give an example to see if I can understand this better. I have a DAC that takes any i2s signal it is getting and automatically does sample rate conversion to 24/96. The DAC chip is capable of 24/192, but is disabled when using this ASRC chip. Let's say I'm playing a 16/44 FLAC file... it is my understanding that the DAC will upsample it to 24/96 on the fly. The same goes for a file that is 24/192... it will downsample it to 24/96. This is just my understanding of how the DAC works.

So if I check the box Decibel will try and change this sample rate of 24/96 to 16/44 and that might be what is conflicting and causing the dropouts I've been hearing. If I uncheck the box, Decibel will try and upsample the 16/44 to 24/96 in software. Am I understanding this correctly?
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Re: Understanding sample rate changes with Decibel and DACs.

Post by sbooth »

What was said in the other post is correct about Decibel's behavior.

There is a device sample rate that Mac OS X uses to talk to the hardware device (nominally over USB, but could be another interface). This sample rate is set using Audio MIDI Setup most often.

When Decibel is not set to automatically switch the sample rate, audio will be resampled to the rate set in Audio MIDI Setup.

When Decibel is set to automatically switch the sample rate, it will attempt to set the device's sample rate to match the file being played. The effect is the same as if you launched Audio MIDI setup yourself and made the change.

Once the audio arrives at your DAC at the sample rate set in Audio MIDI Setup the internal resampling, filtering, etc. is performed.
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Re: Understanding sample rate changes with Decibel and DACs.

Post by Alexk »

In my case, I'm using the optical output directly on the back of a coreduo mini. It's capable of outputs in 44.1/48/96 khz at 16/20/24 bit. I have Decibel set to "adjust sample rate for best quality", and I can watch as my receiver (Sony STR-DA1800-ES) switches up between those three sample rates (it doesn't report bit depth, but I can definitely hear the difference 16 vs 24 back-to-back)

The question is:
  • For sample rates unsupported by my hardware, will Decibel do anything other than factor-2 downsamples?
For example, I'm considering buying a copy of Michael Jackson's Thriller (OK, maybe not the best audiophile album out there, but I don't own it on CD) from HDtracks.com. It's available in:
  • 88.2KHz / 24 bit -$17.98
    • I do have an 88/24 file, and it downsamples to 44.1 (factor 2), but I can't easily tell what bit depth it's outputting (logs don't tell me, but I'm betting it's 24)
  • 176.4KHz / 24 bit - $24.98
    • What would Decibel do if I tried to feed it a 176/24 file?
      • Fail out?
      • Downsample to 44.1 (factor 4)?
      • Downsample to 96 (factor 1.8375)?
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Re: Understanding sample rate changes with Decibel and DACs.

Post by kemoly »

I tested that Michael Jackson thing for you. I have the HDTracks download in 24/176. My setup is an iMac via USB to a Musical Fidelity V-DAC II, with a maximum transfer rate of 96KHz.

Decibel sets the transfer rate to 88KHz and downsamples the track accordingly. Here are relevant log extracts:

Code: Select all

<Info>: Setting output device UID to AppleUSBAudioEngine:Musical Fidelity:Musical Fidelity V-DAC 24/96   :6200000:1
<Info>: Opening output for device 0x43 (Musical Fidelity V-DAC 24/96   )
<Info>: Starting playback for "01 - Wanna Be Startin' Somethin'.flac"
<Info>: Device sample rate (44100 Hz) doesn't match decoder sample rate (176400 Hz)
<Info>: Setting device 0x43 sample rate to 88200 Hz
<Info>: Enqueuing "01 - Wanna Be Startin' Somethin'.flac"
<Info>: Using sample rate converter for 176400 Hz to 88200 Hz conversion
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Re: Understanding sample rate changes with Decibel and DACs.

Post by Alexk »

Well, it occurred to me that a something search might help me locate some test files licensed for free distribution. A dozen or so something searches later, I've got some test files in various formats lined up. Links go to the hosting pages rather than directly to downloads.
  • 44.1 / 16 bit / ALAC / Stereo - ALAC CD quality test tone hosted by Linn Records
  • 48 / 24 bit / WAV / Stereo - TUBULAR 010.wav hosted by freesound.org
  • 88.2 / 24 bit / ALAC / Stereo - ALAC master quality test tone hosted by Linn Records
  • 88.2 / 24 bit / FLAC / 5.1 - FLAC Studio Master Surround 5.1 channel test hosted by Linn Records
  • 176.4 / 24 bit / WAV / Stereo - Dvorak Symphony No 9 ‘From the New World’ hosted by Only the Music
  • 192.0 / 24 bit / WAV / Stereo - Jungle Boldie hosted by Only the Music
I can find and add other combos if others are interested, but I found the above was a good test suite. If anyone can find a good quality 192/24 or better free-to-redistribute file, I could see about asking some producer friends to use their software to resample into various rate/depth combinations so we could compare the same audio source with difference encodings.

To answer my own question:
  • Since my Mac Mini's optical out doesn't support 88.2/24 or 176.4/24, Decibel will downconvert it to 44.1/24 for output to my DAC. You can use the test files above to determine what yours will do!