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.
MIT Online Courseware:
Lecture notes on a variety of digital topics - all free! (Good stuff here!)
Also in Audio Application Notes
Interpolator Overload Distortion
Most digital playback devices include digital interpolators. These interpolators increase the sample rate of the incoming audio to improve the performance of the playback system. Interpolators are essential in oversampled sigma-delta D/A converters, and in sample rate converters. In general, interpolators have vastly improved the performance of audio D/A converters by eliminating the need for analog brick wall filters. Nevertheless, digital interpolators have brick wall digital filters that can produce unique distortion signatures when they are overloaded.
10% Distortion
An interpolator that performs wonderfully when tested with standard test tones, may overload severely when playing the inter-sample musical peaks that are captured on a typical CD. In our tests, we observed THD+N levels exceeding 10% while interpolator overloads were occurring. The highest levels were produced by devices that included ASRC sample rate converters.
Audiophile Snake Oil
The Audiophile Wild West
Audiophiles live in the wild west. $495 will buy an "audiophile fuse" to replace the $1 generic fuse that came in your audio amplifier. $10,000 will buy a set of "audiophile speaker cables" to replace the $20 wires you purchased at the local hardware store. We are told that these $10,000 cables can be improved if we add a set of $300 "cable elevators" to dampen vibrations. You didn't even know that you needed elevators! And let's not forget to budget at least $200 for each of the "isolation platforms" we will need under our electronic components. Furthermore, it seems that any so-called "audiophile power cord" that costs less than $100, does not belong in a high-end system. And, if cost is no object, there are premium versions of each that can be purchased by the most discerning customers. A top-of-the line power cord could run $5000. One magazine claims that "the majority of listeners were able to hear the difference between a $5 power cable and a $5,000 power cord". Can you hear the difference? If not, are you really an audiophile?
Making Sound with Plasma - Hill Plasmatronics Tweeter
At the 2023 AXPONA show in Chicago, I had the opportunity to see and hear the Hill Plasmatronics tweeter. I also had the great pleasure of meeting Dr. Alan Hill, the physicist who invented this unique device.
The plasma driver has no moving parts and no diaphragm. Sound is emitted directly from the thermal expansion and contraction of an electrically sustained plasma. The plasma is generated within a stream of helium gas. In the demonstration, there was a large helium tank on the floor with a sufficient supply for several hours of listening.
While a tank of helium, tubing, high voltage power supplies, and the smell of smoke may not be appropriate for every living room, this was absolutely the best thing I experienced at the show!
- John Siau