Download How Digital Audio Works

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EE-93 Special Topics in Recording Engineering (1 credit)
Thursdays 6 - 7:15 PM, plus 3 - 4:15 TU and THU in Granoff 252
Tom Bates, instructor; George Nagel, teaching assistant
Week 7
SAR A/D converters versus Delta Sigma converters (20 questions vs. running total)
Terms: Oversampled (aliasing frequencies get to be eliminated with digital filters
rather than analog ones, and digital filters have better real-world performance)
Terms: Quantization (quantity; counted as units)
Terms: Digital (digits, as counting on fingers) and Analog (the electrical signal is
analogous to the audio signal)
Master Clock Issues:
Digital audio relies on the principle that the audio is sampled at exact and constant preset
intervals. Otherwise, it could also work well with unevenly spaced intervals if the time
that the sample was taken was also written down in the journal. This doubles the amount
of data that must be written, transmitted, stored, etc.
Digital audio works on the principle that all systems attached to each other and working
together are running synchronously from the same clock.
Jitter
Random timing errors when sampling result in changing timing noise into
voltage noise. (Use chart)
Where timing noise (errors) come from:
A. Voltage noise in comparator switch
B. PLL filter voltage noise and timing noise
C. Charge pump voltage noise
D. Almost everything else
Who should be Master Clock:
Jitter only applies itself permanently to the signal of interest as noise (added to our audio
signal) when the signal is crossing a technological boundary. This is at the A/D or D/A
conversion process. It can also be at a sample rate conversion process, since this digitally
mimics the timing issues of the A/D and D/A conversion process.
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PLL
What is a phase locked loop?
Performance issues (timing errors)
TTL counter for variable slew rate (such as locking oscillator to a musical instrument)
Voltage Controlled Crystal in a 2 stage PLL section
Digital PLL implementation
Dither
Noise added to a small “signal of interest” to make it detectable where it normally
wouldn’t be
Similar to Bias in an analog tape recorder
Somewhat similar to the orange mask on color negative photographic film
People throwing someone or something up in the air in a raft floating on the ocean
Dither actually removes distortion and smoothes low level signals, but adds noise at the
lowest levels. The battle is to remove the most distortion and add the least amount of
discernable noise - which is a trick since the two go hand in hand.
Different types of dither include white noise, different PDF (probability density function)
expressed as triangular or some other shape function, inverse Fletcher Munson curve
filters, UV22 type dither, etc. Each has advantages.
Play Mozart quantization example and then Eugene Friesen.
Inaccuracies and Distortion Products
Analog Signal Distortions:
Harmonic Distortion THD + N – Creation of harmonics (frequencies that are a multiple
of the fundamental frequency in the signal of interest) (plus noise)
Intermodulation Distortion IMD – Creation of sum and difference tones due to the
unintended interaction of two or more frequencies in the signal of interest
Bandwidth/Slew Rate/TIM – Audio filtering of the signal of interest due to the inability
of the circuitry to respond quickly enough at high frequencies, or to sudden transients
Signal Compression/Amplitude Distortion – Unintended level changes in amplitude
(most conspicuous in vacuum tube circuits)
Noise – Unwanted noise added to the signal of interest
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Digital Signal Distortions:
Errors in these digital processes result mostly in noise added to the signal rather than
conventional analog distortion products, although there are some exceptions like clipping.
Last Issues:
Rubber buffers
Double buffered inputs
Digital copies are perfect (unlike analog copies) and may be better than their
originals,
Error correction: Writing down reconstruction material such as sum checks and CRC
(cyclic redundancy checks) so lost material can be reconstituted
SACD vs DVD-A