Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1 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. 2 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 3 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