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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:
44.1kHz from CD
48 kHz from DAT
32 kHz from DSR
One way communication: from a transmitter to a receiver.
Control information:
V (validity) bit : indicates if audio sample is valid.
U (user) bit : user free coding i.e. running time song, track number.
C (channel status) bit : emphasis, sampling rate and copy permit.
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:
Zout: 75ohm +/-20% (100kHz .. 6Mhz)
Vout: 0.4Vpp .. 0.6Vpp, <0.05Vdc (75ohm terminated)
Line receiver:
Zin: 75ohm +/-5%
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 fiber
Connector 3-pin XLR RCA (or BNC)
Signal level 3..10V 0.5..1V
Modulation biphase-mark-code biphase-mark-code
Subcode information ASCII ID text SCMS copy protection info
Max. Resolution 24 bits 20 bits (24 bit optional)
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 is travelling 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 for some reason).
The equipments which can handle both normal audio and IEC61937 just look at those header bits to determine what to do with the received data.
Cabling details
S/PDIF (IEC-958) uses 75 ohm coaxial cable and RCA connectors. 75 ohm coaxial cable is inexpensive, because it is the same cable as used in video transmission (you can buy a video cable with RCA connectors to connect you S/PDIF equipments together). Coaxial S/PDIF connections work typically at least to 10-15 meter distances with good 75 ohm coaxial cable.
AES/EBU-interface uses the well known symmetrical connections with transformer isolation and an output impedance of 110 ohm. The signal-level of this interface is reasonably higher than in the consumer version (3...10 volts). Because AES/EBU digital audio signals are transmitted at high, video-like frequencies (at around 6MHz) and should be handled very differently than standard analog audio lines. Commonly used XLR-3 microphone cables have various impedance ratings (30 ohm to 90 ohm typical) and exhibit poor digital transmission performance. The result is signal drop out and reduced cable lengths due to severe impedance mis-matching (VSWR) between AES/EBU 110 ohm equipment. AES/EBU signal transmission work for few tens of meters with a good cable.
There also an optical version of S/PDIF interface which is usually called Toslink, because uses Toslink optical components. The transmission media is 1 mm plastic fiber and the signals are trasmitted using visible light (red transmitting LED). The optical signals have exactly the same format as the electrical S/PDIF signals, they are just converted to light signals (light on/off). Because high light signal attenuation in the Toslink fiberoptic cable, the transmission distance available using this technique is less than 10 meters (with some equipments only few meters).
What can make difference in the sound of digital signal ?
There are two things which can cause differences between the sound of digital interfaces:
1. Jitter (clock phase noise)
This really only affects sound of the signal going directly to a DAC. If you're running into a computer, the computer is effectively going to be reclocking everything. Same applies also to CD-recoders, DAT tape decs and similar devices. Even modern DACs have typically a small buffer and reclocking circuitry, so the jitter is not so big problem nowadays that it used to be.
2. Errors
This usually causes very significant changes in the sound, often loud popping noises but occasionally less offensive effects. Any data loss or errors in either are a sign of a very broken link which is probably intermittently dropping out altogether.
S/PDIF signals
The signal on the digital output of a CD-player looks like almost perfect sine-wave, with an amplitude of 500 mVtt and a frequency of almost 3 MHz.
For each sample, two 32-bit words are transmitted, which results in a bit-rate of:
2.8224 Mbit/s (44.1 kHz samplingrate, CD, DAT)
3.072 Mbit/s (48 kHz sampling rate, DAT)
2.048 Mbit/s (32 kHz sampling rate, for satellite purposes)
The output impedance is standard 75 ohm, so ordinary coaxial cable designed for video applications can be used. The minimal input level of S/PDIF interface is 200 mVtt which allows some cable losses. There is no real need for special quality cable as long as the cable is made of 75 ohm coaxial cable (a good video accessory cable works also as good S/PDIF cable).
The Coding Format
The digital signal is coded using |
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发表于 2012-2-1 11:30:49
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