What is SPDIF?

noiembrie 15, 2013

SPDIF audio is a digital transmission protocol developed in the 80’s and is based on the standard AES/EBU protocol used in the professional field. If at the protocol level things remained almost unchanged, Sony and Philips transformed  the more expensive professional protocol into a commercial one, this was done by changing the connectors (plugs/wiring) and some hardware specs.

 AES / EBU, S / PDIF (IEC-958)

 Cabling shileded TP 110 ohm 75 ohm coaxial or fiber
 Connector 3-pin XLR RCA, Toslink (or BNC)
 Signal level 3 .. 0.5 .. 1V 10V
 Modulation biphase-mark-code biphase-mark-code
 Max.  Resolution 24 bits 20 bits (24 bit option)

Being developed by Sony in collaboration with Philips the name is no surprise:  Sony / Philips Digital Interconnect Format, hence the acronym SPDIF. (Or, in another version – Sony Philips Digital InterFace)

And if it’s in digital format it helps me?

Audio information stored in our computer or in a CD/DVD  is natively in digital format – and to be used with our speakers have to be transformed into analog electrical signals. This conversion is done by a dedicated hardware chip named: Digital C-to-Analog converter. Conversion scales performance varies depending on the price paid on the hardware. A high-performance DAC provides minimum distortion sound and a value of SNR (Signal to noise ratio) very good.

Digital transmission of sound eliminates digital-to-analog conversion and is noiseless, ie without background noise. So, using a SPDIF connection the sound quality is directly influenced by the receiver’s specifications and audio precincts.

DAC – AKM4396VF fitted sheets Azuzen X-Fi Prelude 7.1 ( see specifications )

Digital - analog converter (DAC)


SPDIF connections are made either through an optical cable (standardized by Toshiba, hence called Toslink) or through a coaxial cable (75 ohm).

TOSLINK optical and coaxial SPDIF cable

Coaxial or Optical?

Although optical connection is generally seen as a superior, in reality there is no difference in the sound quality. Coaxial or optical, it is absolutely the same SPDIF signal, bit by bit identical to the one that is carried on a coaxial connection. One advantage of the optical fiber is that there is no practical limitations concerning the length of the cable. Coaxial connection is limited to a maximum of 10m. Another advantage is that the optical connection is immune to any electrical interference.


Motherboards with SPDIF support (first one were equipped with codec C-Media ® CMI-8738), were almost common in the late 2000’s. Otherwise, SPDIF  was already present in some much pretentious hardware as: Creative Sound Blaster AWE64 (1996), AurealVorex II( 1998) or Creative Sound Blaster Live!(1997).
Meanwhile SPDIF connection has become almost standard among base plate segments among middle and standard sound card.

Optical and coaxial SPDIF on the motherboard

And most laptops made in recent years have SPDIF output. But since SPDIF output is shared with ordinary analog and many users are not aware of its presence. Today SPDIF signal is carried out via a HDMI connection.

Laptop SPDIF

To get a connection via SPDIF output  we need a 3.5mm adapter – RCA, or an RCA jack cable for coaxial transmission.

Whether a mini-plug (3.5mm) – TOSLINK optical connection if. (Or if you already have a TOSLINK cable can buy an adapter that converts one end to form mini-plug)

Toslink mini-plug


SPDIF, Windows, and home theater your home 

With a SPDIF connection we get stereo sound in PCM (uncompressed) or 48kHz maximum compressed sound in Dolby Digital 5.1 (AC-3) or DTS 48kHz with a maximum up to 640kbs bitrate.

Since multi-channel sound format Doby Digital / DTS is present  mostly only in movies, for whatever remains, over the SPDIF connection will get just two channel Stereo sound. This problem has been solved by creating Dolby Digital Live, a techonology which allows real-time creation of an AC3 stream, and this is a good way to have 5.1 sound when playing games.

What is a receiver?

When it comes to SPDIF, a receiver is any hardware that supports this protocol. In terms of audio SPDIF compatibilty, the most popular are home theater receivers. Prices can get very far by fidelity and embedded technologies. The newest receivers are compatible with high definition sound bit/96Khz 24 or 192kHz, and tipically will receive SPDIF signals over a HDMI connection.

Once a SPDIF stream will be detected by the receiver the display will show the corresponding logo:

Dolby Digital and DTS display

Receiver Dolby Digital compatible

Another category of receivers are 5.1/6.1/7.1 systems with digital input. The best example of this is the model of the Logitech Z5500.

Logitech Z5500 featuring Dolby Digital Coaxial Optical Inputs



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