1996-11-08 , E/IC-479/650

Eindhoven, The Netherlands - 8 November 1996 -- Philips was the first company to prove that high quality digital audio was possible over USB when the company showed a prototype USB "Audio Demonstrator" board at the USB Developers Conference early in 1996. Philips Semiconductors has now integrated the Audio board into a single USB DAC (Digital to Analog Converter), the  UDA1321T, which is the first of its kind to be commercially available. This IC takes in the USB signal at the input side and produces a high quality stereo signal at its output. The result is better sound quality, more OEM FLEXibility and ease of use. It also means that sound cards will no longer be required in PCs because most of the basic signal processing capabilities will run in software on the host so system costs will be lowered. Further, it will be possible to control audio features such as volume, bass and treble from the PC through the same USB signal while servicing all the other USB devices at the same time.

The UDA1321T has an embedded  80C51 microprocessor core and provides stereo audio playback over a USB link with an audio sample redistribution module to spread the incoming audio packets over a 1ms frame period. It also provides self synchronisation with the clock for sample frequencies from 5 to 55kHz, i.e. from low quality voice to above CD (44.1kHz) sampling quality.

Separate digital volume control is provided for left and right channels via USB or direct control along with left and right soft mute. Bass and treble tone are digital and effected via the USB. There is also a bypass function that enables the signal to be taken out of the IC for external Digital Signal Processing via standard or Japanese I²S bus serial input format, e.g. graphics equaliser, speaker correction, and other manipulations, and then returned into the chip for the final stages prior to output.

The UDA1321T USB audio DAC is ideal for use in USB monitors, speakers and telephone/answering machines. It is fully compatible with the USB SIE (Serial Interface Engine) and packaged in a 28 pin SO28. The UDA1321T is sampling now with full volume production in early 1997. Pricing is in the region of $6.75 in volume quantities.

The importance of Audio over USB
One of the design aims behind the USB (Universal Serial Bus) concept is that it enables sound processing to be taken outside the PC case, which is inherently an electrically noisy environment and likely to cause interference. With USB, the sound data is kept in digital format right up to the last Digital to Analog Converter (DAC) in the external amplifier. This goes directly to the loudspeaker, giving the extremely high quality hi-fi sound that will further assist the PC in its move into the home environment. As well as resulting in the lack of need for PC sound cards or audio design burden on the motherboard and the associated extra cost, this approach also provides for more than the two channels to which PC sound cards are constrained. Therefore multiple channel surround sound is now possible as all the required data is stored in the digital data stream.

The sound information is sent over the USB through an Isochronous channel, which guarantees the bandwidth on the USB in order to deliver the necessary amount of audio samples in time to a USB audio device, e.g. a USB digital speaker. In an Isochronous channel, a packet of audio samples is sent to the receiving USB audio device every millisecond. On average, each packet contains a number of samples according to the sampling frequency of the signal that is being played. For example, a CD quality signal at 44.1 KHz (i.e. 44100 samples per second) will use packets of 44 samples during 90% of the time and 45 samples during 10% of the time to match the exact sample rate: ~ [44 samples/packet x 0.9] + [45 samples/packet x 0.1] = 44.1 samples per millisecond or 44100 samples per second. Packet positions within each time frame can vary = 1 millisecond time frame = packet of USB data

The receiving USB audio device decodes the incoming USB stream, distributes the "bursty" (i.e. all information concentrated in a packet) incoming audio data stream to a continuous datastream and converts this stream to a high quality analog audio signal.

Philips Semiconductors, a division of Philips Electronics NV, headquartered in Eindhoven, The Netherlands, is the eleventh largest semiconductor supplier in the world. Philips Semiconductors' innovations in digital audio, video, and mobile technology position the company as a leader in the consumer, multimedia and wireless communications markets. Sales offices are located in all major markets around the world and are supported by regional systems labs.