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:
List of 7 items (contains 2 nested lists)
. Audio format : linear 16 bit default, up to 24 bit expandable
. Allowed sampling frequencies (Fs) of the audio:
List of 3 items nesting level 1
. 44.1kHz from CD
. 48 kHz from DAT
. 32 kHz from DSR
list end nesting level 1
. One way communication: from a transmitter to a receiver.
. Control information:
List of 4 items nesting level 1
. 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.
list end nesting level 1
. 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).
list end
Physical connection:
List of 3 items (contains 2 nested lists)
. Cable: 75ohm +/-5% (l<10m) or 75ohm +/-35% (l>10m)
. Line driver:
List of 2 items nesting level 1
. Zout: 75ohm +/-20% (100kHz .. 6Mhz)
. Vout: 0.4Vpp .. 0.6Vpp, <0.05Vdc (75ohm terminated)
list end nesting level 1
. Line receiver:
List of 2 items nesting level 1
. Zin: 75ohm +/-5%
. Vin: 0.2Vpp .. 0.6Vpp
list end nesting level 1
list end
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 the 'biphase-mark-code' (BMC), which is a
kind of phase-modulation. In this system, two zero-crossings of the signal
mean
a logical 1 and one zero-crossing means a logical 0.
_ _ _ _ _ _ _ _ _ _ _ _
| | | | | | | | | | | | | | | | | | | | | | | |
clock 0 ___ _| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_
___ _______ ___ ___
| | | | | | | |
data 0 ___ _| |_______| |___| |_______| |___
signal 1 0 0 1 1 0 1 0 0 1 0
Doc Wright
http://wrightplaceinc.net
>Someday, we'll look back on this,
>laugh nervously and change the subject.
----- Original Message -----
From: "Bruce Toews" <[EMAIL PROTECTED]>
To: "PC audio discussion list. " <Pc-audio@pc-audio.org>
Sent: Monday, January 03, 2005 1:01 PM
Subject: Re: New Sound Card
And what is S/PDIF?
Bruce
--
Bruce Toews
E-mail and MSN/Windows Messenger: [EMAIL PROTECTED]
Web Site (including info on my weekly commentaries): http://www.ogts.net
For information on my weekly radio show visit
http://radioclassics.ogts.net
On Mon, 3 Jan 2005, doc wrote:
> Moving from the firewire port you have
> center channel
> rear channel
> front
> mic
> line in
> s/PDIF
>
>
> Doc Wright
> http://wrightplaceinc.net
>> Someday, we'll look back on this,
>> laugh nervously and change the subject.
> ----- Original Message -----
> From: "Brian Olesen" <[EMAIL PROTECTED]>
> To: "PC audio discussion list. " <Pc-audio@pc-audio.org>
> Sent: Monday, January 03, 2005 11:36 AM
> Subject: Re: New Sound Card
>
>
> Hi,
> First the port looking like a big usb port is infact a firewire port.
>
> The litle lose cable is a digital connection i think.
>
> I don't remember the exact plug, but from the little firewire plug I think
> number 2 is line out 3 microphone and 4 line in, but I'm not quite sure.
>
> Best regards
> Brian
> ----- Original Message -----
> From: "Bruce Toews" <[EMAIL PROTECTED]>
> To: <pc-audio@pc-audio.org>
> Sent: Monday, January 03, 2005 6:47 PM
> Subject: New Sound Card
>
>
>> I just picked up the Sound Blaster Audogy 2 and will be installing it
soon.
>> Can someone tell me about the plugs, please, and the order in which they
>> occur?
>>
>> That little thing that looks like a USB port, is that supposed to be a
>> digital connection? And what is that other little interface that comes
>> with it with what looks like a video port on it? Is it necessary to
>> install it as well? Thanks.
>>
>> Bruce
>>
>> --
>> Bruce Toews
>> E-mail and MSN/Windows Messenger: [EMAIL PROTECTED]
>> Web Site (including info on my weekly commentaries): http://www.ogts.net
>> For information on my weekly radio show visit
>> http://radioclassics.ogts.net
>>
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>>
>
>
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