Transition to Digital Digital Handbook
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TRANSITION TO DIGITAL DIGITAL HANDBOOK Audio's video issues tion within one audio sample, the channels of audio over a fiber-optic in- There can be advantages to locking preambles present a unique sequence terface. This has since been superseded the audio and video clocks, such as for (which violate the Biphase Markby AES 10 (or MADI, Multichannel editing, especially when the audio and Code) but nonetheless are DC -freeAudio Digital Interface), which sup- video programs are related. Althoughand provide clock recovery. ports serial digital transmission of 28, digital audio equipment may provide 56, or 64 channels over coaxial cable or an analog video input, it is usually bet- Like AES3, but not fiber-optic lines, with sampling rates ter to synchronize both the audio and A consumer version of AES3 -of up to 96kHz and resolution of up to the video to a single higher -frequencycalled S/PDIF, for Sony/Philips Digi-24 bits per channel. The link to the IT source, such as a 10MHz master refer-tal Interface Format (more formallyworld has also been established with ence. This is because the former solu- known as IEC 958 type II, part of IEC- AES47, which specifies a method for tion requires a synchronization circuit60958) - is also widely used. Essen- packing AES3 streams over Asynchro- that will introduce some jitter into thetially identical to AES3 at the protocol nous Transfer Mode (ATM) networks. signal, especially because the video it- level, the interface uses consumer - It's also worth mentioning Musical self may already have some jitter. To ac- friendly RCA jacks and coaxial cable.Instrument Digital Interface (MIDI) for broadcast operations. MIDI is es- An early attempt at digital rights management, sentially a protocol that allows elec- tronic musical instruments toex- it is not clear that all sources change performance information. (For correctly set the flag. example, an MIDI command could be"Start_PlayingClarinet_Sound, commodate possible clock differences Despite the primary intent of con- B-flat_above_Middle_C, 50%_Vol- between 59.94Hz and 60Hz video pro- sumer use, the interface also appears ume.") Thus, MIDI is described more cessing, AES3 also provides a flag to in- on some professional equipment. appropriately as a control interface, dicate whether the audio sample rate Although specified for use with 7512 rather than an audio interface. indicated in the channel status shouldcables, many consumers use "plain However, two subsets of MIDI are be multiplied by 1000/1001. vanilla" audio cables for this purpose,useful to know about. One is MIDI Because the AES3 signal has a band- which, over short distances, generallyMachine Control, which provides width of several megahertz, some us-work fine. Optical TOSLINK connec- transport commands for controlling ers may be tempted to use video dis-tors are also sometimes used. recording devices such as multitrack tribution amplifiers to fan out a signal. One key difference between thetape recorders. The other is MIDI While this may work in some applica-AES3 and S/PDIF protocols is theTime Code, which embeds SMPTE tions, a large proportion of video sig-channel status information. The for- time code information in a MIDI nal processing equipment uses clamp- mat (location) of some information stream. While the MIDI specification ing or synchronization circuits thatis different, and AES3 carries much calls for the use of 5 -pin DIN connec- rely on the black level, sync and other more information. Thus, an intercon-tors, nonstandard connectors are oc- components within video for routinenection between the two different in- casionally used on equipment. operation. So, user beware! terfaces could cause problems, if cer- From a physical standpoint, AES3tain equipment needs specific status Optimum performance uses balanced 1100 lines and XLRinformation to function properly. The idiosyncrasies of audio in- connectors, with a nominal signal One flag thatis carried withinterfaces, while somewhat more con- voltage between 2V and 7V peak- S/PDIF but not AES3 is the Serial strained than that of video, still merit to -peak. Coaxial 7511 lines are also Copy Management System (SCMS)attention to maintain the highest sometimes used, as well as cable bun- copy protection info. An early attemptquality plant. Knowing the way these dles (or ribbons) carrying up to 16at digital rights management, it is notinterfaces work can help ensure a lines, terminating in 50 -pin submin-clear that all sources correctly set thetrouble -free installation, as well as iature -D connectors. The modulation flag. Worse still, the system could beprovide ideas for tracking down an used is Biphase Mark Code, whichdefeated by surreptitiously recodingaudio problem - and perhaps, one provides various features, including: the bit stream information. might even implement some useful Clock recovery is achieved easily. When the availability of multi-novel features. BE The DC component (and hence thechannel digital audio recorders made Aldo Cugnini is a consultant in the digital power transmitted) is minimized. multiple circuit cables impractical, the television industry. The interface is insensitive to polar-proprietary Alesis Digital Audio Tape ity reversals. (ADAT) format was introduced in Send questions and comments to: In order to provide synchroniza- 1991 to allow the transfer of up to eight [email protected] 24 broadcastengineering.com I October 2006.