AET 2050 - Daw Production


Conversion Processes

Objectives:


Reading:

Chapter 2.3-2.5, Desktop Audio Technology - Rumsey


Pulse Code Modulation

Analog to Digital Conversion

The encoding section of a conventional stereo PCM recorder consists of:


Dither Generator

The dither generator produces a noise signal that is added to the input so that the effects of quantization error is minimized.

The dither must be added to the signal prior to filtering and digitization. Since it is broadband, it must be filtered along with the signal.

This dither is analog white noise with Gaussian probability - typically, it is 1/2 LSB.


Anti-Aliasing filter (LPF)

A low pass filter must be introduced before digitization to alleviate aliasing.

In a classic recorder (non oversampling), an analog LPF has a cutoff just below the Nyquist frequency with a very steep slope.

S = 48 kHz, Fc = 20 kHz and complete attenuation at 24 kHz.

Limitations:

The small guard band requires the use of a filter with a very steep slope. Such a filter can introduce certain anomalies including:

These anomalies can be overcome with the use of oversampling A/D converters (see below).


Sample-and-Hold (S/H)

as the name implies, has two tasks:

A clock (an oscillator circuit that outputs timing pulses) is set to the sampling frequency.

Some things to consider with S/H circuits:

Limitations:


Analog-to-Digital Converter

Successive Approximation A/D Converter (SAR)

successiveapproximation

SAR examples

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 tests

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:


Digital to Analog Conversion

 


Digital-to-Analog Converter


Sample and Hold


Anti-Imaging (Reconstruction) Filter


Oversampling A/D conversion

Oversampling example

 

 

Advantages of Oversampling:

Noise shaping

Oversampling D/A Conversion

 Oversamling Example (4X ):


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