Audio Myths - The Ultimate "Myth Buster" Video - Monty Montgomery, Xiph.Org

 Digital Show & Tell, Monty Montgomery, Xiph.org

Monty Montgomery takes us into the lab and uses a series of simple demonstrations to bust some very common myths about digital audio. Test your knowledge of digital audio. This video is fun to watch and easy to understand! Monty takes some difficult concepts and demonstrates them in a clear and simple manner.

Have doubts about Nyquist? Have a fear of stairsteps? Are you worried about ringing? Ever wonder what digital audio does to the timing of transients? This video is for you!

This Video Busts the Following Myths:

Myth - "Digital audio has stairsteps." (0:00 - 8:40) - Busted! Monty feeds an analog tone into the A/D and produces a continuous analog tone at the output of the D/A. Watch as he examines the output with an analog oscilloscope and an analog spectrum analyzer.

Myth - "Increased bit depths reduce the stairsteps." (8:41 - 9:36) - Busted! The stairsteps were never really there! Samples are impulses not stair steps. This segment also shows that the word length of a dithered digital system determines the signal to noise ratio and NOTHING else!

Myth - "Analog tape has more resolution than digital audio." (9:37 - 11:34) Busted! Tape noise and gaussian dither noise are virtually identical. Cassette tape is equivalent to about 6 bits, and the best studio reel-to-reel  tapes are equivalent to about 13 bits.

Myth - "Dither masks quantization noise." (11:35 - 13:50) Busted! Dither replaces quantization distortion with a noise spectrum of our choosing. Harmonic distortion (produced by quantization) is removed by dither and this can be seen on the analog test gear.

Myth - "Signals lower than one LSB cannot be reproduced." (13:37 - 14:08) Busted! Watch a 1/4 LSB tone being transmitted without harmonic distortion through a dithered digital system. Dither gives digital systems an analog amplitude response. The video also shows that dither noise can be shaped to reduce the audibility of the noise (14:08 - 14:55). Monty also shows how dither noise is gradually replaced by distortion as the dither power is reduced (14:55 - 16:09). Noise-shaped dither can use a somewhat lower dither power than gaussian dither, without producing significant distortion (16:09 - 17:18).

Myth - "Digital filters make square waves and impulses ring." (17:19 - 20:52) Busted! The ripples are caused by removing high-frequency harmonics, they are not added by the filter. Monty demonstrates this by passing the same signal through two cascaded A/D-D/A conversions.

Myth - "Digital systems cannot resolve timing between samples." (20:53 - 21:55) Busted! The video shows impulses and square waves passing through an A/D and D/A (analog to digital to analog) while delivering continuous resolution of the timing. The sample rate does not limit the timing resolution! Pay particular attention to the segment between 21:47 and 21:55. The red dots represent the digital sampling instances. Notice how the square wave is accurately reproduced no matter what the phase relationship is to the sampling clock. The phase relationship between the sample clock and the transients in the music has no impact on the wave shape that is reproduced. This is counterintuitive, but clearly demonstrated in this analog to digital to analog pass through an A/D and D/A.

More from xiph.org:

This video was Episode 2. Be sure to watch Episode 1.

 

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John Siau
John Siau

Author

John Siau is VP and Director of Engineering at Benchmark Media Systems, Inc.