sampling rate converter
sampling rate converter
Although there arestill dyed-in-the-woolsound technicianswho swear by thegood old analoguerecording technology,most others, bitten bythe digital audiorecording bug, do notwant to revert to analoguerecording. Theyhave experienced thepleasures of loss-freeprocessing and copyingof recordings atdigital level. Thesepleasures turn to frustration,however,when they want toconvert a DAT recordingon to a CD. Thiscannot be done justlike that because thetwo have differentsampling frequencies:DAT 48 kHz, and CD44.1 kHz. To overcomethis difficulty, aconverter as describedin this articleis required.
It is a regrettable fact, with which wewill have to learn to live, that differentaudio techniques use incongruoussampling frequencies (CDI – 18.9 kHz;8mmVCR – 31.5 kHz; NICAM – 32 kHz;CDI – 37.8 kHz; VCR – 44.056 kHz; CD –44.1 kHz; DAT – 48 kHz; and others).The growing popularity of digitalaudio is creating an increasing needof some means of coupling equipmentusing such different techniques– without loss of quality, ofcourse.
This can be done by alteringthe sampling rate in one of the twounits to be coupled, while ensuringthat the two sampled signals are adequatelysynchronized. Clearly, thisrequires a well-designed intelligentconverter.The design of the present converteris based on a dedicated IC: theType TDA1373H from Philips. Thiscircuit is very versatile and may beused for almost any imaginable conversion(but not quite – see later).Thus, it can be used for converting aDAT recording into a CD recording.Also, it enables CD data to berecorded on a DAT machine with asampling rate of only 32 kHz, which,of course, results in a much longerplaying time. Another possibility isconverting the consumer standardS/PDIF* to the professional AES/EBU*format. True, the converter has noAES/EBU connectors, but the conversionis possible.Apart from as a converter, the circuitmay be used as a copybit eliminator.In that case, the two sampling ratesare made equal (as in the converter),but the category code, the copybit andthe generation-status bit are set.
Thesampling rate used must correspondwith the code set in most DAT machinesto ensure that the signal is accepted.Finally, the circuit may also serve asjitter killer since the first-in-first-out(FIFO) and gain stage in the TDA1373Hsuppresses any jitter.D E S I G NThe circuit of the converter is shownin the diagram of Figure 1. Circuit IC1is the integrated digital converter, IC2is the controller, and IC3 is the outputinterface.