Digital Audio

Quantization

(amplitude component) translates the voltage levels of a continuous analog signal into binary digits (bits).

What is done to prevent aliasing?

1. A low-pass filter with a cutoff at Nyquist frequency (brick-wall) is placed into A/D circuit. 2. Raise sample rate

A sample rate of 20Hz will have a Nyquist Frequency of ____ and successfully record frequencies up to _____.

10 Hz, 10 Hz

signal-to-error ratio formula

6n + 1.8 dB

Using signal to error ratio, what is the dynamic range of a 24-bit word?

6n + 1.8 dB 6(24) + 1.8 dB= 144 + 1.8 dB= 145.8 dB

Anti-alias filter

A filter that stops frequencies that are greater than half the sample rate frequency (that create distortion) from passing into A/D conversion.

Why are higher bit resolutions better?

Adds internal headroom at the bit level and help to reduce errors in level and performance at low-level resolutions. reduces erros that might accumulate within the LSB

____ is the most common technique for statistically minimizing quantization error.

Dither

What is better- high or low sample rate? why?

High sample rate allows for more samples to be taken more frequently, therefore providing a more accurate composite of original source.

Why is Nyquist theorem useful?

If frequencies greater than twice the sample rate are allowed into sampling process, the samples won't be able to accurately capture higher frequencies and will record "alias" (false) frequencies, heard as harmonic distortion.

Nyquist theorem

In order for the desired frequency bandwidth to be faithfully encoded in the digital domain, the selected sample rate must be at least twice as high as the highest frequency to be recorded.

Why is Word Clock necessary?

It serves as a master clock between individual timing components in the digital audio chain. Otherwise, unwanted noises happen.

How are the edges of "staircase" waveform within a D/A converter rounded off to yield an acceptable output waveform?

LPF smoothing filter (smooths edges)

Which transmission formats use TOSLINK connectors?

S/PDIF and ADAT

Explain the relationship between sample rate and frequency response.

Sample rate relates to the resolution at which a sound will be recorded. It is determined by the overall frequencies to be capture. Higher sample rate typically increases frequency response.

Be able to draw a picture of an LSB close-up on an amplitude axis. Show a waveform and sample period on the time axis, and showing explanation for quantization error.

See pg. 208 and test 2.

Be able to draw a diagram of a PCM signal

See test 2

When does aliasing occur?

When frequencies are higher than twice the sample rate, samples are not accurately captured, creating "false" frequencies heard as harmonic distortion.

S/PDIF

a consumer-level variation of the AES/EBU digital audio protocol

TDIF

a signal format for transferring up to 8-channels digital audio between Tascam digital multitrack recorders or digital mixers

Jitter

a time-based error caused by varying time delays in the circuit paths from component to component.

What is dither?

addition of very small amounts of randomly generated noise to an existing bitstream.

How is Word Clock implemented?

all devices in a digital audio system have to synchronize to a single, common 'master' word clock source - which can be any of the devices in the network, or a separate external word clock generator.

What is dither used for?

allows low-level signals that are below LSB to be encoded at less than the LSB (minimizes quantization error)

ADAT Lightpipe

an 8-channel digital audio format developed by Alesis. This signal format uses TOSLINK optical connectors

TOSLINK

an optical digital audio format developed by the Toshiba Corporation

the sample rate of a system determines its overall ______.

bandwidth

What digital audio conversion parameter is responsible for dynamic range size/resolution?

bit depth

Explain the relationship between bit-depth and dynamic range.

bit depth is directly related to the number of quantization steps in the bitstream. Therefore, quantization is related to signal-to-error, and overall dynamic range. Higher rates increase headroom. If the depth is too low, quantization errors will occur, leading to distortion. Too high of a depth might create a very large file.

SCMS

digital protection flag

signal-to-error

indicates the degree of accuracy that's used to capture a sampled level and its step-related effects.

A poorly shaped edge on a transmitted pulse signal can be classified as ______.

jitter

_____ can be minimized by using cables with proper impedance ratings.

jitter

worldclock

master timing reference that maintains consistent timing for the overall sample-and-hold conversions within the system, so clicks, pops, and jitter are avoided.

What does the Sample and Hold element in an A/D converter do, and for what time duration?

measures analog voltage level that's present during the sample period. The period is determined by sample rate.

bit depth/resolution

number of bits (0's and 1's) that determines number of available values for quantization

Dither consists of a _______ type of signal at an amplitude in the ____ range.

random noise, LSB

Sample value bits can be used to generate an analog "staircase" waveform with a network or "ladder" of _______.

resistors

A recording made at a higher sample rate will be capable of ____.

storing a wider range of frequencies.

MADI

supports 56 channels

Headroom

the difference between highest signal level of input and the saturation (distortion) level. Basically, room you have left to play with.

Sample rate

the number of measurements (samples) that are periodically taken over the course of a second. (time element)

AES/EBU

transmission protocol that conveys 2 channels of digital audio through a single XLR or line cable in a single direction. Most common transmission.