Technology Update Readyfor digitalaudio? It mightarrive sooner than you think Andy Rappaport, AssociateEditor

By the end of this year, you might be buying hourJong stereo records that fit in your pocket. Not only will these disks be small, but their manufacturers claim they will offer lower distortion, wider and flatter frequency re- sponsethan conventionalLPs. You won't be able to play the records on your present turntable, though-these first commercial products of the digital-audio revolu- tion will have no grooves and will spin much faster than 33/s rpm. Although the digital-audio tech- nology behind such devices isn't new, the high cost and complexity of the required conversionand storage have confined its application pri- Onfy 4.7 in. across, the Compact Disk (CD) holds as much as t hr of digital stereo marily to professional equipment. audio. CD players should be available in the US by the end of this year. Now, however, as data converters' prices drop as fast as their exists, but record companieswill be end of this year, is basedon a 4.7-in. accuraciesimprove and as practical reluctant to invest in expensive (12-cm) optically encoded disk. high-density storage media become digital-disk or tape-production facil- Similar to the larger Sony video- available, you'll see a proliferation ities that they feel might become disk, the CD record contains a of low-cost consumer-orienteddigi- obsolete.Furthermore, operatorsof 2.5-milong helical track of 0.6-pm- tal-audio products. The develop- recording studios, where audio wide pits and flats spaced1.2 pm ment of these deviceswill not only signalsare first digitizedand mixed, apart-allowing 8x10e active bits radically changethe audio industry, will delay spendingthe 9200,000to per record. As it's scanned,the disk but will also affect general data $300,000 to equip their setups alternately reflects and disperses a acquisition and signal processing. until a standard is developed. solid-state-laserbeam. To circumventthis problem,some The Sony/ system uses a Still no standards equipment manufacturers are en- 16-bit linear-encodedpulse-code- One of the biggest problems now tering cooperativeagreements with modulation (PCM) format with a confronting digital-audio-equipment record producers. The first such 44.3-kHzsampling rate. Sony claims designers is a lack of industry venture, between Sony (Tokyo) and the system achieves more than standardization. Committees in CBS/Japan (Tokyo), entails a joint 90-dB dlmamic range and channel Japan, Europe and the US are commitment to produce the Com- separation with less than 0.05Vo discussingthe subject, but none has pact Disk (CD) systemdeveloped by total harmonic distortion (THD). It yet made a satisfactory recommen- Sony and NV Philips (Eindhoven, compares these specs with the dation. Hardware manufacturers Netherlands). Both Sony and Phil- 35-dB channel separation, 55-dB expect that the first system to gain ips will manufacture players, and dynamic range and 0.2Vo THD widespread consumer acceptance CBS will releasea limited catalogof typical of an analog LP record (see will establish a de-facto standard- disks. The two equipment makers box, "Are digital specs as good as one the industry is forced to adopt. hope that availability of the CBS they look?"). This approach,however, results disks will establish the feasibility of The CD player eliminates wow in the classic chicken-or-egg prob- the CD system and that other rec- and flutter'by servoing the disk's lem: Manufacturers won't sell ord companieswill join the fold. speed to maintain a constant data enough machines for a standard to The CD system, which Sony rate. The player comparesincoming develop unless sufficient software expectsto releasein the US by the data with an internal crystal i EDN FEBRUARY3, 1982 6l Technology Update

reference, and the resulting error digital-audio-disk player, capable of Philips, other Japanese companies signal controlstheunit's motor. The generating 5 hrs of programming on have developed digital-audio-disk optical bit length remains constant two sides. It feels that the smaller players. Of the 16 that demonstrat- throughout the record's.track; thus, system, which plays for t hr using ed machines at the recent Tokyo the player must maintain constant only one side, represents a better Audio Fair, most use the CD linear velocity by decreasing disk choice because it's cheaper and format. A few, however, are rotation speed.as the laser scans matches the analog LP's program- experimenting with systems of outward from the record's center. ming format. Moreover, the CD their own. system is small enough for use in JVC, for example, is working on Car CD system coming cars, and Sony promises an automo- an audio adapter, termed AHD, for Sony has stopped development of tive system soon. its VHD Series videodisk systems. its previously demonstrated 12-in. Not to be outdone by Sony and An AHDA/HD combination can

Until takes over... Digitalaudio is coming,but don't throwaway your to produce.As a result,record companies will be slow presentgear -you'll have to convertit gradually. in issuingdigital disks, and these records initially will First,you'll need to investabout $800 to $1000in a be 50 to 100% more expensive. digital-audio-diskplayer. Manufacturershope that But while you'rewaiting for digitalaudio to take theseunits will become less expensive, although they over,you can benefitfrom analog-audio innovations. feelthat even the costlierearly units will be no more Twocompanies, for example,have introduced analog expensivethan analog turntablesof comparable signal-processingsystems that allowconventionally quality. manufacturedLPs to otfer-at lowercost-the same Second,you won't see too many digitalrecords dynamic range as the digital disks. Both firms' soon.Optical disks require totally ditferent manufac- techniquesinvolve compressing a signal'sdynamic turing processesthan analog LPs, so to produce range as a record is cut and then restoringthe them, recordcompanies will have to make large signal'soriginal characteristics during playback. In- investmentsin plantsand equipment.Furthermore, steadof requiringa completelynew player, listening the digitalrecords are similarto videodisks,which, to these recordsdemands only that you buy an althoughsimple in concept,have proven very difficult inexpensiveprocessor that connectsto yourcurrent

RIGHT OUTPUT

A 4-flfter array provides the time constanfs in CBS3 CX decoder. A deadband in the rectilier circuit prevents expansion of very-lowlevel signals. Encoders are formed by putting the decoder in an op amp's feedback loop.

EDN FEBRUARY3. 1982

I Technology Update

display still video pictures as well as Lake City, UT) has also proposed a 3x3-in. record to hold as much as 1 provide stereo audio. No marketing different type of digital-audio play- hr of stereo audio. Moreover, plans for the unit have yet been er. Its system employs a moving because high-resolution frlm proc- announced. optical scanner that reads a essing is a well-established art and Sources within the industry also stationary record. The fixed-record photographic materials are inexpen- expect RCA to introduce some t5pe design, the firm claims, results in a sive, a record for the firm's machine of audio device for its CED Series less complicated and potentially could be made for only $1. This low videodisk players, but no specifics cheaper system than the Japanese cost, coupled with simple process- are available. spinning-disk units. irg, should make the system says that its attractive to low-volume record Fixed record, moving reader unit packs as many as 3x 108bits/in.2 producers, according to the sys- Digital Recording Corp (Salt on silver-halide film, allowing a tem's designers.

preampor recever. CBS licensesthe decodercircuitry to equipment The GX system, recently developedat CBS manufacturersfor a smallfeeand provides encoders TechnologyCenter (Stamford,CT), allows record to interestedrecord producers.A few companies companiesto makeanalog LPs from digitalmaster havealready signed licensing agreements, so CBS tapeswith no loss of dynamicrange. When played anticipatesthat CX decodersand recordswill soon througha CX decoder,the encodedrecords otfer a be available. 20-dBhigher signal-to-noise ratio than conventional Untilthe systemgains widespread acceptance, its LPs. In addition,CBS claimsmost listenersnotice developerspredict, some existingrecords will be onlya minorloss of soundquality when playing back reissuedin the CX format,and new releaseswill be .theCX diskswithout a decoder. availablein bothnormaland encoded form. Addition- The CX encodercompresses by 2:1 the dynamic ally someJapanese manufacturers have expressed rangeof all signalsexcept those more than 40 dB interestin incorporatingCX circuitryinto their receiv- below a referencelevel; the compressionslope ers,and to makethese plans economically feasible, convertsrapidly to 1:1 below -40 dB. CBS claims threesemiconductor houses are reportedworking on thatbyleaving very low signals unaltered, the system decoderlOs. preventsmaster-tape hiss from becoming louder than A second analog-disk-enhancementsystem, of- record-surfacenoise. fered by Inc (Newton,MA), also employsa To minimizethe etfectsof CX compressionon compression/expansiontechnique. In this system, recordsplayed back withouta decoder,the encoder though,all signalsare compressed2:1 regardlessof maintainsflat frequency response. Besides providing level.Input signals are rms-detectedand processed adequateaudio quality for undecodedrecords, CBS with a single time constant.In additionto signal claims that a wide-bandprocess best suits the compression,the encoder provides 12-dB pre- requirementsof record-noisereduction. Although emphasis,which, according to DBX, helps reduce record noise increasesat the audiblefrequency recordtic-and-pop noise. extremes,human-hearing sensitivity to low levels DBXclaims that any encoding process that audibly decreasesat low and high frequencies,so the changesa musicalsignal requiresthe use of a perceiveddisk noise spectrum is flat. companiondecoder. Unlike CBS, then, it expectsits Unlike conventionalcompression/expansion recordsto be playedback only through a decoderand schemes,the CX systemdoesn't merely detect a thus isn't concernedwith the undecodedsound of signal'srms level.lnstead, the processorfull-wave- encoded-onlydisks. This approach,DBX explains, rectifiesand peak-detects the input signal and passes enablesits systemto providegreater noise reduction the resultingamplitude-dependent voltage through withsimpler circuitry than the CX system. four parallelfilters (figure). Each filter providesa Toget the systemoff the ground,DBX is manufac- specificattack or decaytime constant. The system's turing and selling encoded records. lts catalog designersclaim that the 4{ilter array allows fast currentlyincludes more than 200 titles, including responseto musical-levelchanges while avoiding the workspurchased from 40 recordcompanies. To play audible noise pumping common to other audio thesedisks, you can use DBX'sModel 21 decoder companders.To ensure that the encoder'sfilter (approximately$110) or the firm'stape-noise-reduc- characteristicsexactly matchthose of the decoder, tion systems. DBX predictsthat as the system the encoderis constructedby placingthe decoder catches on, receivermanufacturers will integrate circuitin the op-ampfeedback loop. disk-decodingcapabi lity intotheir products.

EDN FEBRUARY3, 1982 Technology Update

Digital Recording'sengineers large German record manufacturer But the DACs employed in home also suggestthat their player is well and is ready to make and market digital-audio players must cost only suited to such automated audio records itself if necessary. a few dollars each and be available applications as Musak and prepro- in considerablevolume. To this end. grammed radio broadcasting. The Impact on industry Sony claims that its Semiconductor square records suit stacking in a Although home disk players are Div has developedan inexpensive sliding cube, allowing either se- not the only digital-audio products 3-chip set that performs all of the quential or random access. on the way, they are the onesmost 16-bit linear PCM encoding and Although no definite plans for the likely to have the greatest impact decodingfunctions. It has no plans fixed-record player have yet been on other areas of electronics. For to offer the deviceson an OEM basis announced,Digital Recording says instance,the use of 16-bit DACs is but expects it will have to supply it's currently building six engineer- currently confined to a small them to other audio-disk manufac- ing prototypes, and full-scale pro- segment of the data-acquisition turers to spread digital technology. duction could begin within 2 yrs. field. These devices bre usually Analog Devices (Norwood, MA) Because the records are easily designed into high-accuracyindus- already offers its Model 4D7546, a produced, the firm doesn't see trial and commercial instruments, CMOSmonolithic 16-bit DAC, which software as a big problem. It has where their relatively high prices it says suits the needs of digital already establishedrelations with a have not poseda problem. audio. Realizing that the part's

Are digital specs as good as they look? Judgingby specsalone, digital audio looks terrific: mumsignal level depends solely on itsword size, but Designersof 16-bit systemswith 44- to 50-kHz an analog recorder'speak amplitudecapability clockingrates claim flat frequency response from 20 dependson recording-headand tape-saturation char- Hz to 20 kHz,90-dB dynamic range and -80-dB acteristics.Good analog tape decks offer more than harmonicdistortion. However, some audioexperts 1O-dBheadroom above the O-Vu reference level. Les disputethese claims. Tyler of DBX Inc (Newton,MA) claimsthat these Forinstance, P B Fellgett,writing tor StudioSound factors,plus the use of noise-reductiontechniques, (November1981), points out that the dynamic range allow analogrecorders to exhibitalmost 110-dB of an N-bitdigital system is 20log(2N-1),or approxi- usabledynamic range. mately96 dB for N:16 bits.This ratiois usually Digitaldistortion figures, too, are subject to debate. comparedto 70 dB typ for a 15-ipsanalog tape The lowestTHD in a digitalsystem occurs at the recorder.But the figurescan be misleading-the highestsignal level, and distortion increases as the digitalspec reflectsthe differencebetween a sys- signal amplitudeis reduced.By contrast,analog tem'slargest and smallest possible words, while the systemsdistort large signals most, and THD at low audiospec indicates the ratioof tape noiseto 0 Vu, levelsdisappears into the noise.Thus, comparisons an rms-sine-wavereference level. madebetween digital and analogsystems at 0-Vu Tomake a truecomparison, Fellgett contends, you signallevels are misleading.Additionally, Fellgett mustconsider realistic conditions. A sinewave's rms claimsthat at lowlevels the stepdiscontinuities that levelis about9 dB belowits peak-to-peakvalue. A digitalsystems produce are similar to theundesirable 16-bitdigital system, then, which produces 1-LSB crossovernotches that Class B amplifiersgenerate. noise,actually provides only 87-dB dynamic range. John Curl of JC Designs(San Francisco,CA) Fellgettfurther explains that the humanhearing arguesthat the introductionof digital audio is mechanismcan discernsignals that lie belowthe reminiscentof the developmentof solid-statesys- wide-bandnoise level. Psychoacoustic experiments tems in the mid-'60s.Then, transistoramplifiers have demonstratedthat the ear acts as a wave appearedto achievemuch better performance than analyzer,comparing signal level only to the noisethat tube designs,but they generateda completelynew surroundsit in a narrowband. Unlike digital systems, set of distortionproblems, Curl contends that it has in whichno signalscan lie belowthe noise,analog taken morethan a decadeto identifysome of the recordingscontain discernible information buried in moresubtle effects of "transistorsound," and digital thenoise floor. The ability of the hearingmechanism audiowill most likely undergo the samescrutiny. to recognizethese signals, Fellgett contends, adds Of course,where audio is concerned,the truth is in 16to 20 dB to an analogsystem's dynamic range. thelistening, and it won'tbe longbefore you'll be able Tape-saturationlevels also affectan analogsys- to decidefor yourself. tem'sdynamic capability. A digitalsystem's maxi-

EDN FEBRUARY3. 1982 Technology Update initial price of$20 (100)is still a bit too high for mass-market home players, the firm is working on increasing production efficiency. Higher demand for 16-bit DACs, it explains, should bring prices down. Burr-Brown (Tucson, AZ) has also introduced converters aimed specifically at the audio market. Its recently announced PCM50 16-bit DAC is the companionto the PCM75 17-psec16-bit ADC. This voltage- output DAC offers an internal reference and starts at $50 (100). Steve Harwood, Burr-Brown's con- verter-products marketing manag- er, claims that the extremely rapid development of digital-disk players has causedhis company to acceler- ate the evolution of its DAC line. and the converters it plans to fntended for digital-audio use, these Burr-Brown converters offer 16-bit accuracy and short introduce shortly will bear much conversion times. Non-audio usersol parts like these will benefitfrom digital-audiotechnology lower price tags. by being able to take advantage of the improved performance and reduced prices of high-resolutionAID and DIA Although DAC prices are already converters. dropping in anticipation of increased Production of records for home and stored on hard magnetic disk. demand, though, converter manu- digital players requires that record- The editor holds as much as 34 min facturers expect digital audio to ing studios finally switch to a digital of stereo audio and can accommo- have only a small effect on ADCs. format. Although studio-quality date any clock rate from 44 to 50 As Dave Kress, new-products digital tape decks have been offered kHz with words to 16 bits. marketing manager at Analog by a few companies since the Unlike conventional analog edi- Devices, points out, early home mid-'70s, their high cost and the tors, which require that splices be digital-audio systems will offer lack of adequate digital-editing made in order from a tape's playback capability only, so audio facilities have slowed acceptance. beginning to its end, the Sound- ADCs will be used primarily in Indeed, Tom Stockhamof Sound- stream device allows editing passag- professionalrecorders. Therefore, a stream Inc (a subsidiary of Digital es in any sequence. The editor's single ADC will feed tens of Recording Corp) suggests that operator can assign labels to specific thousandsof DACs. ADC users then recording studios have had limited words within the stored data, and will benefit from the improved incentive to upgrade to digital gear. these labels, or timed offsets from perforrnance of the audio convert- He claims that although digital them, define splice locations. ers, but prices of these parts will recorders offer far better sound As a program gets edited, the not change as rapidly. quality than analog machines, the machine stores all Computer and industrial mass- improvements gained by upgrading splice information in a playback storage applications should also to the digital decks are masked by map. The original data never gets benefit from the development of the deficiencies of analog LP altered, allowing simultaneousgen- digital-audio devices. Unlike video- records. Now that data on digital eration of multiple maps. Thus, a tape and videodisk systems, audio master tapes can be transferred recording engineer can easily com- records store unforrnatted data, directly to digital disks, though, pare different edited versions of a free from the constraints ofline and Stockham feels that studio opera- stored musical passage. frame s;rnchronization. An optical tors will be more anxious to invest Stockham claims that by allowing disk capableof holding 1010bits thus in new equipment. such ease of operation, audio offers an excellent alteraative to To simplify the production of devices such as the Soundstream magnetic media in read-only appli- digital tapes, Soundstream has editor will not only provide im- cations. For example, some soft- introduced what Stockham claims is proved sound quality, but will also ware vendors hope to eventually the first practical digital-audio help record producers perfect their market home-computerprograms.in editor. Programs to be edited are art. EDX optical-disk form. loadedinto the editor in digital form

66 EDN FEBRUARY 3, 1982