User manual DCS (AUDIO) PURCELL

[. . . ] dCS Purcell Digital to Digital Converter User Manual Software Release 2. 2x September 2004 © dCS Ltd. 1999 - 2004 Price UK £17. 50 / Euro 25. 00 All rights reserved. No part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted in any form, or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of dCS1. Any person who does any unauthorised act in relation to this publication may be liable to criminal prosecution and civil claims for damages. [. . . ] · · · · · · SPDIF outputs The SPDIF messaging is based on the IEC60958 standard and is implemented as follows. · · · · Consumer. Copy prohibit flag is passed through from an SPDIF input. Emphasis flag is determined by the input and the De-Emphasis menu setting. Emphasis flag is determined by the input and the De-Emphasis menu setting. Wordlength is coded. 10 11 AES3-1992 (ANSI S4. 40-1992) "AES Recommended practice for digital audio engineering ­ Serial transmission format for two-channel linearly represented digital audio data. ", available from the AES. "dCS 24/96 and 24/194 Coding Formats. " 1997, available from dCS on request. Manual filename: Purcell Manual v2. 2x. doc English version Page 51 email: more@dcsltd. co. uk web-site: www. dcsltd. co. uk dCS Purcell User Manual dCS Ltd Manual for Software Issue 2. 2x September 2004 GENERAL TECHNICAL INFORMATION Wordlength Reduction Wordlength reduction (truncation) causes an error signal to be added to the wanted signal. The error signal is usually referred to as "Q noise" or Quantisation noise ­ the approximation is made that the errors are noise like. This is true for large signals, but for smaller ones it is not so. As the wanted signal gets smaller, the complexity of the error signal decreases, and the errors first of all pile into ever lower order harmonics or intermods, and then, as the level of the signal sinks below the Q level, much error power piles into the fundamental. This causes its amplitude to become unpredictable ­ it may drop abruptly to zero and disappear, or it may cease to go down any more and just stay at a constant level. From the audio viewpoint, this sounds very unpleasant. As a signal tail decays away, the tonal quality changes, and then it decays into distorted mush and either abruptly stops, or else keeps fuzzing away until a new signal starts. The level at which all this happens is the lsb of the output word ­ for CDs, it is at the 16 bit level, which equates to about -90dB0. The level is high enough to be quite audible, and the effect must be tackled to make reasonable quality CDs. There is really only one way of tackling the problem ­ another signal has to be added to the wanted one to smooth the staircase transfer function that truncation causes. Mathematically, with two signals present, the transfer 12 function the wanted signal sees is the convolution of the PDF of the second signal and the staircase function. The converse is also true ­ the transfer function the additional signal sees is the convolution of the PDF of the wanted signal and the staircase function. This aspect is not a problem with the dither types considered below, but it can be with some highly frequency shaped dithers. The additional signal is usually referred to as dither, and it is usually noise-like, because then its statistics can be controlled, and the converse effect of the signal modulating the dither can be made insignificant, or zero. However, there are a number of ways that this dither signal can be generated and treated. The major options are: · generate from the signal or generate independently and add ("Dither"). [. . . ] 3 upsampling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 23 upsampling spectrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 upsampling, improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 using DSD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [. . . ]