Digital Formats & Protocols Explained

There are various digital audio formats, or protocols, used within the Pro Audio and Broadcast industries. This Blog provides a brief overview and explanation of the four main variants, namely SDPIF, AES/EBU, TDIF & ADAT.

Note that whenever one connects two or more products via any of the formats listed above (i.e. SPDIF, TDIF, AES/EBU or ADAT), there must be one word clock master, which all the other products slave to, please refer to Blog covering Synchronisation. For reference, all of the digital audio protocols covered within this blog actually contain word clock information within the digital audio stream, so simply setting slave units to the appropriate format and sample frequency normally suffices. However, within more complex setups, users may need to use BNC word clock connections to control overall synchronisation.

SPDIF

SPDIF is simply an abbreviation for Sony Phillips Digital Interface Format. An SPDIF delivers a stereo digital (L-R) audio signal via a single lead. The connection may be either co-axial in nature, e.g. a phono to phono cable, or optical, which requires a Toslink cable. Although ideally co-axial cables with specific impedance are required, I have found from experience that any good quality phono lead will generally suffice.

SPDIF can be found on a number of products from mastering products such as the Tascam MD350 and Tascam CDRW900 to audio interfaces, e.g. RME Fireface 800 and Presonus Firepod.

AES/EBU

SPDIF is essentially a consumer format of AES/EBU. AES/EBU is an abbreviation for Audio Engineering Society/ European Broadcasting Union, which is a professional digital interface format, or protocol and although very similar to SPDIF it actually uses higher signal voltages.

AES/EBU is a Balanced Co-axial connection and delivers a stereo digital audio (L-R) signal via one XLR-XLR cable.

TDIF

TDIF is an abbreviation of Tascam Digital Interface format, which is a protocol designed to deliver a 16 channel digital audio stream (8 send and 8 return) via a co-axial connection, which is in the form of one 25-pin D-Sub cable.

Note: Due to bandwidth limitations, one TDIF connection can only support bit widths of 16 to 24 at sample frequencies ranging from 44.1 kHz or 48kHz if passing 16 channels of digital audio. The channel count is halved if running sample frequencies of 88.2kHz or 96kHz, i.e. two TDIF ports will be required to pass 8 channels of I/O at either 88.2kHz or 96kHz.

ADAT

ADAT, which stands for Alesis Digital Audio Tape, was a protocol designed for the Alesis ADAT digital recorder that recorded 8 channels of digital audio to a VHS tape. This protocol has way outlived the ADAT machines and is commonly used within Pro Audio and Broadcast applications for passing multi-channel digital audio streams. In short, ADAT may be thought of as a Toslink or optical equivalent to TDIF and two Toslink cables are required to send and receive 8 channels of digital audio.

Note: Due to bandwidth limitations, one ADAT pair can only support bit widths of 16 to 24 at sample frequencies ranging from 44.1 kHz or 48kHz if passing 16 channels of digital audio, i.e. 8 I/O. The channel count is halved if running sample frequencies of 88.2kHz or 96kHz, i.e. two ADAT ports will be required to pass 8 channels of I/O at either 88.2kHz or 96kHz.

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2 Responses to Digital Formats & Protocols Explained

  1. Andrew Toomer says:

    Thanks for the explanation. I’m wanting to use an ADAT protocal (via a Toslink cable) from my HTPC and into an outboard DAC (8 channels out). The soundtrack on Blueray discs are a maximum 24bit/48khz which I understand is movie industry standard. How many ADAT ports will be required to carry the sound? I’m guessing just one should suffice…

    Cheers

  2. Paula says:

    Hey! Great blog! I was wondering if you are acquainted with the ADAT standard protocol in a technical matter.. I´m trying to implement a Digital audio stream decoder in a FPGA VIRTEX-4 CI , and I really wanted to have 8 mono-signals as inputs. I thought of ADAT as a great option but I´ve tried to look for some kind of support in the web of how it actually works, with no results at all. I´m not even sure if it is an open protocol, because I´ve tried to look for an owner or the like with, once again, no results. If you know anything related to this topic , I would appreciate your help A LOT. Thanks.

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