ePanorama.net - Audio DocumentsSPDIFHistoryCharacteristicsThe interfaceMulti channel audio and S/PDIFCabling detailsWhat can make difference in the sound of digital signal ?1. Jitter (clock phase noise)2. ErrorsS/PDIF signalsThe Coding FormatWord and Block FormatsChannelstatus and subcode informationElectrical InterfaceConverting between AES/EBU and S/PDIF interfacesWhat are different types of IEC 958-interfaceJitter specifications of AES/EBU interfaceConversion circuitsHow to do different conversions using the circuit belowAES/EBU to S/PDIF signal level converterS/PDIF to AES/EBUS/PDIF conversion circuit building blocksS/PDIF output buffer circuitsS/PDIF output buffer circuitAnother S/PDIF output circuitOutput circuit with transformer used as impedance matcherOutput with 1:1 pulse transformerSimple S/PDIF output without galvanic isolationSimplest TTL to S/PDIF coax interfaceIsolation transformer for S/PDIF outputS/PDIF input circuitsS/PDIF coax input circuitAnother input circuitOptical S/PDIF output circuitOptical TOSLINK outputTORX173 and TOTX173 module pinoutsVery cheap optical S/PDIF outputOptical S/PDIF output circuitOptical TOSLINK receiverTORX173 and TOTX173 module pinoutsOther related informationWho makes AES/EBU digital audio transceiver chips ?Information sourcesePanorama.net - Audio Documents SPDIF History Since the early 80's, a step towards digital audio has been set by the introduction of the Compact Disc player. In the beginning, those signals stayed inside the set, and were converted to analog signals before leaving the cabinet. A new trend is to keep signals into the digital domain as long as possible, because this is the only way to keep the signal quality. To make this possible different devices must be able communicate with one another within the digital domain. Several interfaces exist to perform such tasks, from which one has grown to the audio standard worldwide: IEC958 1989-03 (consumer Part) from the EBU. In Japan an equivalent EIAJ CP-340 1987-9 is standard. Characteristics Standard IEC958 "Digital audio interface" from EBU (European Broadcasting Union) details: • Audio format : linear 16 bit default, up to 24 bit expandable • Allowed sampling frequencies (Fs) of the audio: o 44.1kHz from CD o 48 kHz from DAT o 32 kHz from DSR • One way communication: from a transmitter to a receiver. • Control information: o V (validity) bit : indicates if audio sample is valid. o U (user) bit : user free coding i.e. running time song, track number. o C (channel status) bit : emphasis, sampling rate and copy permit. o P (parity) bit : error detection bit to check for good reception. • Coding format: biphase mark except the headers (preambles), for sync purposes. • Bandwidth occupation : 100kHz up to 6Mhz (no DC!) • Signal bitrate is 2.8Mhz (Fs=44.1kHz), 2Mhz (Fs=32kHz) and 3.1Mhz (Fs=48kHz). Physical connection: • Cable: 75ohm +/-5% (l<10m) or 75ohm +/-35% (l>10m) • Line driver: o Zout: 75ohm +/-20% (100kHz .. 6Mhz)o Vout: 0.4Vpp .. 0.6Vpp, <0.05Vdc (75ohm terminated) • Line receiver: o Zin: 75ohm +/-5% o Vin: 0.2Vpp .. 0.6Vpp The interface IEC958 is a newer standard which supersedes AES/EBU and also S-PDIF. The S/PDIF interface (IEC-958) is a 'consumer' version of the AES/EBU-interface. The two formats are quite compatible with each other, differing only in the subcode information and connector. The professional format subcode contains ASCII strings for source and destination identification, whereas the commercial format carries the SCMS. Here is a short comparision table of AES/EBU and S/PDIF interfaces: AES/EBU S/PDIF (IEC-958)Cabling 110 ohm shileded TP 75 ohm coaxial or fiberConnector 3-pin XLR RCA (or BNC)Signal level 3..10V 0.5..1VModulation biphase-mark-code biphase-mark-codeSubcode information ASCII ID text SCMS copy protectioninfoMax. Resolution 24 bits 20 bits (24 bitoptional)NOTE: AES/EBU also exists in 75 ohm/BNC version (AES-3id-1995 standard). 75 ohm BNC version of AES/EBU is very electrically similar to 75 ohm coaxial S/PDIF shown above. The electrical characteristics of AES/EBU are based on on RS-422, so basically any differential RS-422 chip will do as the receiver and transmitter chips. S/PDIF coaxial interface is not specificaly based on any other standard I know of (but is quite similar in signal levels and bandwidth requrements to some video signals). Both S/PDIF and AES/EBU can, and do transfer 24 bit words. In AES/EBU, the last 4 bits have a defined usage, so if anyone puts audio in there, it has to go to something that doesn't expect the standard specifies. But in S/PDIF, there's nothing that says what you have to use the bits for, so filling them all up with audio is acceptable. Typical S/PDIF equipments only use 16 or 20 bit resolutions. While many equipments use more than 16 bits in internal processing, it's not unusual for the output to be limited to 16 bits. Note on HDR-2 (2 pin header) interface used in some PC products: Many modern PC CD-ROM drives and some soundcards (SB32, AWE32, etc.) have a two pin digital output connector in the back of the drive and they sometimes call that interface S/PDIF. Unfortunately the electrical signal which comes from it is not exactly what is described in S/PDIF specifications. The data format is exactly the same, but the signal is TTL level (5Vpp) signal instead of the normal 1Vpp signal. The output level might be selected to make the interfacing to other digital electronics easy when signal istravelling inside the computer (the normal output driver system and input amplifiers can be avoided). The downnside of this is that you need to build some electronics to make the signal from the CD-ROM drive to match what normal S/PDIF equipments expect. Multi channel audio and S/PDIF IEC958 was named IEC60958 at 1998. IEC60958 (The S/PDIF) can carry normal audio and IEC61937 datastreams. IEC61937 datastreams can contain multichannel sound like MPEG2, AC3 or DTS. When IEC61937 datastrams are transferred, the bits which normally carry audio samples are replaced with the databits from the datastream and the headers of the S/PDIF signal. Channel-status information contains one bit (but 1) which tells if the data in S/PDIF frame is digital audio or some other data (DTS, AC3, MPEG audio etc.). This bit will tell normal digital audio equipments that they don't try to play back this data as they were audio samples. (would sound really horrible if this happens
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