This adds noise and distortion.
? Droop – The decrease in voltage as the capacitor leaks between samples.
Analog-to-Digital Converter
The A/D is the key to the encode side of PCM digitization and is the most critical component in the entire electronic system. Any errors introduced by the A/D will follow the audio signal throughout the remaining signal chain and ultimately back into its analog state.
Essentially, the circuit must:
? Examine the sampled input signal
? Determine the quantization level nearest the sample?s value
? Output the binary code that is assigned to that level
Two important qualities in an A/D converter are:
? Speed – (conversion time – the time it takes to output a digital word) must perform a complete conversion within the span of one sampling period. For example, S=48k, 48,000 conversions per second per channel.
? Accuracy – quantization intervals must be evenly spaced throughout the entire amplitude range.
Successive Approximation A/D Converter (SAR)
Successive approximation is one of the earliest and most successful analog-to-digital conversion techniques.
This conversion process utilizes a “divide and conquer” approach:
? The digital data is gathered one bit at a time by comparing the measured value to a reference value.
? As each comparison is made, the reference value is divided in two, making a smaller comparison window.
? When all bits are tested, the digital word represents the closest approximation possible to the original measurement.
The result of this process is a PCM representation of the measured value.
An example with a 4-bit system:
Input voltage = 13
1000 = 8 (DA conversion)
Is 13 => 8 (comparison)
Yes, 1100 = 12
Is 13 => 12
Yes, 1110 =14
Is 13 => 14
No, 1101 = 13 (reset)
Is 13 => 13
Yes, 1101
13 = 1101
The number of bits defines number of checks
Another example with an 8-bit system:
Input voltage = 6.92
10000000 = 5 volts
Is 6.92 > 5
Yes, 11000000 = 7.5
Is 6.92 > 7.5
No, 10100000 = 6.25 (reset)
Is 6.92 > 6.25
Yes, 10110000 = 6.875
Is 6.92> 6.875
Yes, 10111000 = 7.1875
Is 6.92 > 7.1875
No, 10110100 = 7.03125 (reset)
Is 6.92 > 7.03125
No, 10110010 = 6.95312 (reset)
Is 6.92 > 6.95312
No, 10110001 = 6.91406 (reset)
Is 6.92> 6.91406
Yes, 10110001 = 6.91406
6.92 = 10110001
The number of bits defines number of checks
Limitations:
The repetitive process of measuring and comparing must be done as many times as there are bits, for each sample measured, regardless of the sample’s value.
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