S/PDIF adapter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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Recently, AV receivers and active speaker systems with a digital audio signal input according to the S / PDIF standard have become widespread. In addition, the home computer is increasingly being used as a media hub, as well as for storing movies and music. Many modern computer motherboards already have S/PDIF connectors. This interface allows you to transmit multi-channel audio over a single coaxial or optical cable, eliminating the need for a separate shielded cable for each audio channel. Often, the computer and the active speaker system are separated by more than 2 m, and the S/PDIF standard allows you to transmit a digital stream of audio signals that is resistant to interference and pickups, as well as provide galvanic isolation of the equipment (when using an optocoupler).

For a coaxial connection, you must use a coaxial cable with a characteristic impedance of 75 ohms. These requirements are met by the cable used to connect the TV antenna. With an optical connection, the devices must be connected with a standard optical fiber connection system - TOSLINK.

It is well known that a running computer is a source of a wide range of interference that adversely affects the quality of the output signal (both audio and video). Most of this interference occurs due to the galvanic coupling of the computer with the output device.

If the AV receiver contains a galvanic isolation of the coaxial cable, then you are lucky and you only need to limit the output signal level from the motherboard [1]. The S/PDIF transmission standard limits the signal amplitude to 0,5 V. You can check for galvanic isolation by ringing the coaxial input screen contact and, say, the analog input screen contact. If the resistance is infinite, then there is galvanic isolation.

If the coaxial input of the AV receiver does not have galvanic isolation, then it is preferable to connect the computer and the speaker system with fiber optics, which will reduce interference and eliminate the risk of damage to the inputs / outputs of devices during accidental "hot" switching on / off. If you need to connect at a distance of 10 meters or more, you will need to find a rare and expensive TOSLINK cable. In addition, a bracket with an optical S / PDIF output is not always included with the computer motherboard. Therefore, it is more convenient and cheaper to make a communication line on a simple twisted pair, and directly at the optical input of the acoustic system, convert the electrical signal into an optical one using an LED.

The S/PDIF interface installed on the motherboard usually has an S/PDIF Out signal with TTL levels. A simple LED output load with a current-limiting resistor can be suggested. In this case, the pulsed current may exceed the permissible output current of the microcircuit installed on the motherboard and responsible for the output of the S / PDIF signal. In my case, the nominal current of the LED is 20 mA, and the allowable current of the ALC889 chip is 10 mA. This option is fully functional, but does not guarantee trouble-free operation and can lead to equipment malfunction, as it overloads the output on the motherboard. Therefore, for reliable operation of the interface, I propose to make an adapter.

I used such an adapter to connect the SVEN-HT500 active speaker system and a computer at a distance of 10 m. optical connector S / PDIF speaker system.

Fig. 1

The adapter diagram is shown in fig. 1. The S / PDIF signal from the corresponding motherboard connector is inverted and amplified by six buffer elements of the DD1 chip with increased load capacity. The output current of one element in the state of logic zero can reach 24 mA. Resistors R1 and R2 are needed to limit the current of the LED and protect against short circuits. The resistance R1 and R2 may need to be selected so that the adapter works stably when the LED is removed at a distance of 10 ... 20 mm from the optical connector. As an LED, it is optimal to use a super-bright one with a wavelength of 640 ... 660 nm. As DD1, any TTL chip with sufficient load capacity and speed required to transmit a signal with a frequency of up to 6 MHz can be used. The adapter scheme must be changed accordingly.

Rice. 2 Fig.3

On fig. 2 shows a drawing of the adapter printed circuit board with dimensions of 35x15 mm. Capacitors are installed by surface mounting (C1 is soldered to the microcircuit pins).

The L-813SRC-F LED is desirable to be modified according to fig. 3 in order to install it directly into the optical connector of the audio equipment.

Fig. 4

On fig. Figure 4 shows a photograph of the adapter being placed on a standard computer bracket. To do this, it is necessary to drill a hole for the RCA connector in it. Both pins of the connector must be insulated from the bracket and the computer case.

Now you can enjoy Dolby Digital and DTS stereo or multi-channel audio when watching DVDs, which will be reproduced by your AV receiver if it is equipped with the appropriate decoders. Note that only a stereo (PCM) stream, such as a music CD, AC3 (Dolby Digital) or DTS stream, can be fed to an external AV receiver. That is, you can enjoy watching a movie on DVD or listening to your favorite CD. But most toys with multi-channel (but not Dolby Digital) sound on the receiver will not be able to listen. To do this, you need a sound card that allows you to encode any sound into a Dolby Digital stream on the fly and send it in this form to the digital output. You can also use the AC3Filter program, which allows you to decompose any sound source into a multichannel one [2]. Otherwise, you will need to use an analog connection to the AV receiver to get multi-channel audio in games.

Considering the wave impedance of the twisted pair, which is close to 100 ohms, it is better to place the resistor R2 on the load side of the twisted line, near the HL1 LED.

Literature:

1. SPDIF (AES/EBU to S/PDIF signal level converter). - epanorama.net/documents/audio/spdif.html.
2. AC3Filter & SPDIF. - ac3filter.net/ guides/ac3filter_spdif.

Author: A. Kharlov, Dimitrovgrad, Ulyanovsk Region; Publication: radioradar.net

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