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Identifying Digital Audio Errors: Testing and Solutions

Explore the challenges of digital audio errors, testing methods, and potential solutions outlined in a joint technical symposium. Discover the limitations of current software tools and the critical importance of aligning and testing stereo pairs to ensure audio accuracy. Gain valuable insights on null testing, error size calculation, and the impact of varying bit depths on audio quality. Learn about the experiences of testing multiple components simultaneously and the detailed analysis of digital audio interstitial errors for a comprehensive understanding of audio fidelity.

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Identifying Digital Audio Errors: Testing and Solutions

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  1. Bits is Bits? Right?Check Again. George Blood, Safe Sound Archive Joint Technical Symposium Oslo, Norway Monday, May 3, 2010

  2. Typical Playback Chain Analog source Analog-to-digital converter Digital Audio Workstation Digital File Physical Carrier of Files

  3. Typical Playback Chain Analog source Analog-to-digital converter } Digital Audio Workstation Digital File Physical Carrier of Files

  4. From ADC to Digital File ADC Digital Output (AES-3) Cable/patching/switching Digital Audio Workstation: Interface: AES-3 to FW/USB/etc. Driver Computer: CPU/Bus/etc. Software OS/Drivers Storage interface/drivers: FW, USB, SATA, FC, iSCSI Storage Device (HDD) Digital File

  5. Disappointments • Don’t mention brands • Don’t tell you our solution • Very pervasive problem • http://www.avpreserve.com/wp-content/uploads/2010/01/Digital_Audio_Interstitial_Errors.pdf

  6. Disappointments • Planned to announce a software tool to automate this testing….

  7. Tested Hardware • Lynx AES16 • M-Audio (96 and 192) • M-Box • Metric Halo ULN-2 • Nagra V • SonicStudio 302 (aka ULN2+DSP)

  8. Tested Software • Boom • Metric Halo Console • ProTools LE (Mac) • Sonic Solutions HDSP • Sonic Studio soundBlade • SoundForge • WaveLab

  9. Tested OS • Mac OS9 • Mac OS X • Windows 98 • Windows Home • Windows ME

  10. Tested Drivers • Stock drivers (mostly) • OS stock drivers • Hardware bundled drivers • Dedicated Drivers • OS 9 (Sonic Solutions) • OS X (SonicStudio & MH)

  11. Tested Storage • Fat32 • FibreChannel (2Gb, RAID5) • FireWire 400 • HFS+ (SCSI Ultra160) • HFS-extended • SATA

  12. Not Tested • Audacity • AudioCube • [Bias Peak] • iSCSI • ProTools HD • ProTools LE (PC) • Pyramix • RME • ADC to FireWire

  13. Testing Multiple Components Simultaneously • A b 1 • A b 2 • B b 1 • B b 2 • A a 1 • A a 2

  14. Finding errors • 100% Listen • Inefficient • Unreliable • Software verification • Doesn’t certify authenticity • Limited parameter testing • Unreliable

  15. Finding Errors • Null Test: • 2 presumed identical files • Sample-accurate alignment • Invert polarity • Add together • If a1=a2, then a1 - a2=0

  16. Aligning 2 stereo pairs (1)

  17. Aligning 2 stereo pairs (2)

  18. Aligning 2 stereo pairs (3)

  19. Aligning 2 stereo pairs (4)

  20. Aligning 2 stereo pairs (5)

  21. Aligning 2 stereo pairs (6)

  22. …then invert 1 pair

  23. Testing for 2 parameters • Time • Number of samples • Amplitude • Number of bits

  24. Null if mis-aligned by 1 sample

  25. Error of 76 samples dropped

  26. from: http://www.avpreserve.com/wp-content/uploads/2010/01/Digital_Audio_Interstitial_Errors.pdf

  27. Bits vs. samples missing • 1010101010101010 • 1010101011101010

  28. Bits vs. samples missing • 1010101010101010 • 1010101011101010

  29. Null if gain changed

  30. What happened? • New Hardware • Failure • Reviewed set up with engineer • Failure • Reviewed old systems • Failure • Simplified Test • Finally some passes • Failures continue between identically configured systems

  31. Qualifying a Control • Round Robin Test • A≠B • B≠C • A=C • …B is wrong

  32. Qualifying a Control • Assumes only one variable at play • Host • I/O Hardware • Driver • Application …test configuration

  33. Hardware in hand • SonicStudio • Great editor • Semi-proprietary file types • Non-destructive editing • No support for WAV or BWF output • No AES-31 ADL support • Nagra V • Simple • Self-contained • Can’t change variables • (other than resolution)

  34. Results • Null tests passed • Every time • Always • Validates testing procedures

  35. Talent Pool • Head of QA with physics background • 2 experienced audio engineers • 2 dealers • 2 outside technical engineers

  36. Tech “support” • Universally appalling • Short attention span • Don’t understand concepts • Always some other link in chain • Regional sales reps care more • Engineer curious but unable to help

  37. Computer code “traps” • Intractable problem buried in code • Build routine to watch for it • Applies fix • Efficient solution

  38. Computer code “traps” • Masking (aka “hiding”) problem! • Little crossfades at glitches • Other lies…

  39. True Nulls

  40. Very small error in Null

  41. Very small error in Null Close up

  42. Incomplete Null

  43. Incomplete Null Non-zero offset

  44. Calculating size of error • “Normalize” difference signal • Increases gain to zero headroom • 1 bit = ~ 6dB • Normalizing amplitude divided by 6dB equals # of bits (amplitude) of difference…

  45. Calculating size of error • 90dB / 6 dB = 15 bits

  46. Incomplete Null Spectrum of difference signal

  47. Conclusion • Not passing 24 bits • Truncating at 16 bits • Dithering 16th - 24th bits • More noise but Lower distortion than truncating at 16 bits and dithering 17-24

  48. Struggling to keep up Different bit depth, all non-zero Example 3 drops samples

  49. Struggling to keep up Example 1 & 2 Normalized -- “properly” dithered

  50. “Well-dithered lie”

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