Glossary

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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

 

 

A

 

       AES3 - A two-channel digital audio interface standard, as provided at the dScope's Digital Output and Input.  Also known as AES/EBU this format is used in professional applications usually with balanced XLR connections.  It carries audio wordlengths up to 24 bits, plus Valid bit, User bit, Channel Status and Parity bit per channel.

 

       AES11 - An AES3 carrier used as a Reference Sync rather than to carry an audio signal.  Also known as 'DARS' (digital audio reference signal).

 

B

 

       Balanced - A method of transmitting an analogue audio signal or digital audio carriers, where two wires are used each carrying a representation of the signal or carrier in opposite polarity.  Receiving equipment extracts the signal by subtracting one 'leg' from the other, thus rejecting any signals common to both wires.  In this way, interference from mains, radio communications etc. is rejected, assuming the CMRR of the receiving equipment is adequate at the appropriate frequency.  See also Unbalanced.  dScope's Analogue Inputs and Outputs can work in balanced or unbalanced modes.

 

C

 

       Channel Status - status information embedded in an AES3 or S/PDIF digital interface, one bit per channel per sample-period, which accumulate into a 192–bit frame for each channel every 192 sample-periods.  The frame is arbitrarily split into many fields of various lengths, with diverse functions as described in the appropriate interface standard document.  The definitions of the fields and their meanings are different for 'Consumer' Channel Status (where the first bit of the frame is 0, used in S/PDIF) and 'Professional' Channel Status (where the first bit is 1, used in AES3). Originally conceived to add functionality to digital equipment interconnects, the proliferation of outputs with sloppy Channel Status implementation and inputs which mute if any unexpected Channel Status is received has led, like the Babel Fish, to much entirely unnecessary conflict.

 

D

 

       DARS - see AES11.

 

       dBFS - decibels with respect to digital full scale.  0dBFS is defined as the RMS amplitude of a sine wave whose peak reaches a positive full scale sample value (0x7FFFFF Hex).

 

       Dither - low amplitude noise, added to a signal before quantization, or re-quantization, to linearize the loss of precision. In the dScope, dither is applied by default to Digital Outputs. Best linearization is achieved by TPDF dither.

 

E

 

F

 

       fs - Abbreviation for sample rate.

 

       FS - Abbreviation for digital full-scale, e.g. dBFS

 

G

 

H

 

       High-pass filter - A filter in the Continuous-Time Detector and FFT Detectors of the dScope's Signal Analyzer which eliminates frequencies below a pre-set limit from the measurement.

 

I

 

       Interface jitter - Jitter present on a digital audio carrier or reference sync.  Interface jitter usually comprises fs jitter and data jitter components.  The dScope can generate and measure interface jitter directly, and incoming interface jitter can also be demodulated for analysis by the Signal Analyzer.  Interface jitter is often blamed for sonic degradation in A/D and D/A converters, but this is usually due to sampling jitter within the conversion equipment resulting from the equipment failing adequately to remove incoming interface jitter from the conversion clock.  Where good quality converters are used, interface jitter is not usually problematic until it reaches very high levels, when data loss can result.  The AES3 standard defines a jitter tolerance template (jitter vs frequency) for correct receipt of data.

 

J

 

       Jitter - Variation in edge-timing of a clock.  In audio systems, manifestations are interface jitter and sampling jitter.

 

K

 

L

 

M

 

N

 

O

 

P

 

Q

       

R

 

       Reference Sync - A signal passed between digital audio equipment for the purpose of defining the sampling clock.  It is usually in AES11, Wordclock or video format.  The dScope can accept a Reference Sync for its Digital Outputs in any of these formats.

 

S

 

       S/PDIF - A two-channel digital audio interface standard, as provided at the dScope's Digital Output and Input.  This format is used in consumer applications usually with unbalanced RCA (phono) or optical (TOSLINK) connections.  It carries audio wordlengths up to 24 bits, plus Valid bit, User bit, Channel Status and Parity bit per channel.

 

       Sample-rate - The rate at which a digital audio signal has been sampled.  Standard sample rates include 32kHz, 44.1kHz, 48kHz, 88.2kHz, 96kHz, 176.4kHz and 192kHz.

 

       Sampling jitter - Caused by jitter present on the sampling clock of an A/D or D/A converter (or a sample-rate converter).  Sampling jitter results in distortion of the converted audio, which is worse at higher frequencies.  In practice, sampling jitter often occurs in conversion equipment which does not adequately remove interface jitter from its reference sync.  Sampling jitter is usually measured by passing a high-frequency tone through the converter under test, and applying jitter of varying frequency to its reference sync.  Sampling jitter is manifest by side-bands on the converted tone, and the variation in amplitude of these with varying jitter frequency enables the jitter rejection characteristic of the conversion equipment to be measured.    

 

T

 

       TPDF - Triangular Probability Distribution Function – A noise function, where a graph of probability vs amplitude is triangular. This type of noise is often used for dithering re-quantizations in digital audio, since it produces a precisely linear transfer function.

 

U

 

       Unbalanced -  A method of transmitting an analogue audio signal or digital audio carriers, where a single wire carries the signal with respect to a ground, or screen conductor.  This method is more common in consumer equipment and is more prone to interference than the balanced method commonly used in studios. dScope's Analogue Inputs and Outputs can work in balanced or unbalanced modes. See the Unbalanced operation and grounding section for more details.

 

       User bits - A per-channel, per-sample status bit in the AES3 interface.  There are many different User bit implementations in use, some pseudo-standard (such as CD and DAT sub-codes and AES18 data transmission) and others which are entirely proprietary.

 

V

 

       Valid bit - A per-channel flag bit carrier in the AES3 interface.  The meaning of the Valid bit has changed slightly since the AES3 standard was originated, so unfortunately its implementation sometimes differs between equipment. In general, it indicates (when 0) that the channel is suitable for conversion to analogue, and most receiving equipment mutes if it detects the flag set to 1.  However, in some instances it has been used to indicate that error correction or concealment has taken place, the effort of which may have been wasted if receiving equipment mutes as a result of seeing the flag. The dScope can set the Valid bits in its Digital Outputs and monitor them at its Digital Inputs.

 

W

 

       Wordclock - A Reference Sync signal in the form of an unbalanced square clock at the sample rate.  It is nominally at TTL level with 75R impedance, on a BNC connector.

 

X

 

Y

 

Z