In order that the F3 series CD player, the CD20R, sets the standard for technical ability and sound quality, there were a diverse set of skills and experience that had to be applied to improve the state of the art. This paper describes the technical features incorporated in the design of the CD20R Compact Disc player.

Introduction
CD has been with us since 1982, when it was first launched, and has found its way into other applications which have now outgrown the original concept as a digital audio carrier. Whilst CD promised perfect reproduction, due to its digital nature, it is only in recent years that its full potential has been realised due to technology improvements and a better understanding of digital audio. TAG McLaren Audio have made use of this increased knowledge and new technology in the development of their CD20R CD Player.

The CD20R is a high quality CD player that can also be used as a CD transport. However, it is not optimised as a stand alone CD transport as is the case with the CDT20R. The player is designed for absolute precision, both measured and auditioned, and utilises RF circuit techniques and audiophile components to achieve this. The following sections discuss some of the issues of the reproduction process, and how they have been handled in the development of the CD20R. The block diagram in figure 1 shows the key components that are discussed.

The Compact Disc
To appreciate the advanced technology used in the TAG McLaren Audio CD20R, it is best to start by looking at the engineering problems faced in extracting the data from the compact disc.

The Compact Disc has a spiral track of pits from which the information is read. Unlike an LP, the readout is by means of an optical, non-contact, process. The pits are about 0.5 µm wide, 700 times smaller than a pin prick, and the pitch between the tracks is 1.6 µm. The spiral runs from the centre to the outer edge of the disc, with a a length of around 5 km (20,188 revolutions). The data is extracted from this spiral using a laser to generate an eye pattern, the reflected beam is then read by the optical pick up and decoded using a CD decoder.

To generate an eye pattern from which all the data on the disc can be retrieved, the laser spot should follow the centre of the 0.6 µm wide track to within about ± 0.1 µm, with no interference from adjacent tracks just 1.6 µm away. Since the track on the disc may be slightly eccentric due to tolerances in the disc pressing equipment, and to handle the effects of vibration, the servo control should furthermore be able to accommodate a maximum side to side track swing of about 300 µm. The requirements for the focusing servo are equally severe. For adequate read-out, the focusing servo has to keep the laser beam focused on the reflecting layer of the disc to within ± 0.5 µm, even with a maximum disc warp of 1 mm.

These requirements are fully met in the CD20R by using a high quality CD mechanism along with digital servo control.

CD Mechanism
The CD20R uses a Philips CDM12.4 mechanism, which is specifically designed for CD-ROM applications where data integrity is of the utmost importance. The mechanism has the photo diode pre-amplifier and laser supply control circuit on the flex foil, close to the detectors, to ensure the signals leaving the flex foil have a good signal to noise ratio. This is in contrast to the CDM12.1, recommended for cost effective solutions, which requires these components to be mounted on the main PCB, a significant distance away from the detectors.

Servo Control & CD Decoding
Servo control is performed using the OQ8868 digital servo controller from Philips. This part is an improved version of the TDA1301, offering lower noise in the focus loop, and improved jump performance. The digital servo removes the need for external trims and does not suffer from changes in component tolerances, thus ensuring consistent performance at all times. The CD decoding is carried out by an SAA7345, also from Philips. This part performs all the necessary decoding and makes full use of the CD’s Cross-Interleaved Reed-Solomon Code (CIRC) to both detect and correct errors, thus ensuring a reliable data stream for conversion back to analogue.

Prerequisites for Accurate Digital to Analogue Conversion
In order for the original signal to be reproduced accurately from the digital data, it is essential that the data is error free and the clock used for the conversion process is precisely timed. Clock purity is a particular area where great attention to detail is paid in all TAG McLaren Audio digital audio products. It ensures that the reproduced audio is precise in every detail, making the experience life-like.

Master Clock
The CD20R adopts a discrete design for its 33.8688MHz master clock. This is in contrast to using the recommended oscillator configuration around the SAA7345, like most other designs. The oscillator is designed for high Q, low noise performance, and being separate from the SAA7345, is immune to data induced modulation. The SAA7345, along with a divide by two circuit, are fed from the buffered output of this oscillator via AC coupling.

DAC
The CD20R employs the CS4329 from Crystal Semiconductor for the digital to analogue conversion. The part incorporates a 128 x digital interpolator, fourth-order delta-sigma modulator, 1-bit DAC, and analogue (switched capacitor) low pass filter. Attention to design detail and PCB layout around the DAC stage ensures that text book performance is achieved. Power supply de-coupling and digital signal routing (clocks & data) are just a couple of the items given special consideration in the design.

Analogue Filters & Muting Circuits
The output filter is designed for good attenuation in the stopband whilst maintaining a constant group delay (linear phase) in the passband. The design makes use of OPA134s from Burr Brown, and high accuracy, low temperature coefficient, Vishay metal film resistors. These components ensure both accurate filtering characteristics and audiophile performance.

The output stage is direct coupled, using a servo to remove DC offsets. The servo provides a low frequency break point of around 0.1 Hz which is essential for maintaining the phase information of transient signals. The servo amplifier is an OPA2134, a dual OPA134, one amplifier for each channel.

The final output stage provides a low impedance (100R) drive to the phono output sockets, thus allowing long cable lengths to be driven. The outputs are also stable for all capacitive loads, making them suitable for use with all types of audiophile interconnects.

Digital Output - SPDIF
One SPDIF (IEC958) digital output is provided on the CD20R to allow connection to either a digital recorder or outboard DAC. The output drive stage is AC coupled to the phono connector and is designed to present an accurate 75 W impedance to the outside world.

User Interface
The CD20R has been designed to have a simple, easy to use, minimal key press user interface. The front panel supports all the basic functions whilst the remote control provides for the less used ones and has a numeric keypad for direct track selection. Programming, with extensive edit facilities, is also provide to allow a sequence of user selected tracks to be played.