24-Bit DACs to Improve DVD Sound
The chips will enable audio systems to deliver the finest fidelity ever, as well as supporting the audio standard for DVD players and drives.
By May 1998, digital-analog converter (DAC) integrated circuits (IC) offering 24-bit resolution and sampling frequencies of 96kHz, will begin to reach the market (Table 1). These chips, known simply as 24-bit DACs, will be in volume production by five firms. Analog Devices, Inc and Crystal Semiconductor will begin producing the chips in the US; in Japan, Asahi Kasei Microsystems Co, Ltd and Burr-Brown Japan, Ltd will manufacture the chips; and Philips Electronics NV of the Netherlands is the only European firm to announce production plans.
The new 24-bit DAC will offer the highest-ever resolution, delivering the finest audio fidelity available: with a peak signal/noise (S/N) ratio of 120dB (Fig 1), and a peak dynamic range of 115dB. The sum of total harmonic distortion plus noise (THD+N), an indicator of overall DAC performance, is only -102dB.
The theoretical S/N for the 24-bit DAC is about 144dB, but in general, ICs are thought to have a practical S/N limit of 120dB. The 24-bit DAC shipped in 1998 will approach this limit quite closely.
For example, Burr-Brown Japan, a subsidiary of Burr-Brown Corp of the US will ship a DAC with an S/N of 120dB in spring 1998. The DAC from Asahi Kasei will have an S/N of 115dB (Fig 2). Crystal Semiconductor, a business arm of Cirrus Logic, Inc of the US, is developing a 24-bit DAC with a target S/N of about 120dB.
Performance will not be the only improvement in 1998. Other models will also be available: low-priced chips at ¥200 or under, multichannel designs, and chips operating with very low supply voltages.
Complying with DVD-Audio Standard
The reason that the 24-bit DAC volume production stance has firmed up so quickly is that the chips are expected to be widely adopted by digital video disk (DVD) players. The audio standard for DVD players specifies 24-bit resolution linear pulse code modulation (PCM) with a peak sampling frequency of 96kHz. DVD players released before the first half of 1997 did not comply with this standard, however, because 20-bit DAC chips were the highest resolution available.
DVD players shipped at the end of 1997 finally adopted 24-bit chips, supporting the high-fidelity standard in an effort to shift DVD player sales into the fast track. This approach was used by firms such as Matsushita Electric Industrial Co, Ltd of Japan, Victor Co of Japan, Ltd (JVC), and Pioneer Electronic Corp of Japan.
There are other standards in the works that will provide even better audio. DVD-Audio, for example, is a standard being drawn up for audio-only large-capacity optical disks. This standard will use a sampling frequency of 192kHz, and the DAC ICs needed to implement the standard are currently under development.
Also Aimed at High-End Audio Systems
The industry is also moving to use the 24-bit DAC in compact disk (CD) players and CD-equipped mini-component stereo systems for the audiophile. Companies are replacing existing 20-bit chips with 24-bit products especially in the high-end systems priced at hundreds of thousands of yen more. Kenwood Corp and Sansui Electric Co, Ltd, both of Japan, for example, exhibited prototype CD players with 24-bit DACs at Audio Fair '97, held in Tokyo in November 1997. JVC released a non-portable model in December 1997 with 24-bit DACs.
"Marketing Bit" for AV Systems
The 24-bit DACs designed for use in audio-visual (AV) equipment are called "marketing bit" products. While the resolution has been boosted from 20 to 24 bits, the S/N and dynamic range are in fact slightly inferior to existing 20-bit DAC chips.
A segment-type 20-bit DAC, for example, offers an S/N of about 120dB and a dynamic range of about 110dB. The sigma-delta-type 24-bit DACs shipped in 1997, however, offer peak S/N and dynamic ranges of only about 106dB each. The complexity of the DAC circuitry rises sharply with the increase in resolution, making interference between different circuit blocks more common. Without new measures to reduce noise, the 24-bit designs actually deliver performance below that of the 20-bit chips.
The 24-bit chips to be shipped in the spring of 1998 will offer dramatic improvements in both the S/N ratio and the dynamic range.
Shipments will start for both voltage-output sigma-delta designs and current-output segment-type 24-bit DACs. In 1997 a single sigma-delta-type chip supporting 24 bits and 96kHz was available in three models. Segment-type chips will be added in 1998, increasing the total number of products available to two-type designs and four models. Sigma-delta chips will offer up to 115dB for S/N and dynamic range both, while segment chips will offer an S/N of 120dB and a dynamic range of 112dB.
Key Features: S/N, Price, Vcc
The semiconductor manufacturers have four major aims: (1) improved performance, as expressed by S/N and dynamic range, (2) lower price, (3) lower supply voltage, and (4) multichannel design.
Of these, Asahi Kasei Microsystems is pursuing improved S/N through the sigma-delta design, while Burr-Brown Japan is implementing high performance through the segment design which is expected to boost S/N more than sigma-delta design. Crystal Semiconductor is concentrating on lower prices. And Philips is developing ICs for mobile phones that support six channels and supply voltages of +2.7V.
Sigma-Delta Offers S/N of 115dB
The 24-bit sigma-delta-type DAC has finally attained an S/N better than 100dB. The AK4393 from Asahi Kasei Microsystems, for example, offers an S/N of 115dB, which represents a significant improvement from the 98dB of the AK4324 shipped in 1997. Compared to the previous model, the new chip reduces operating noise by 17dB (seven times).
Different manufacturers are using different methods to boost S/N. In the sigma-delta DAC, S/N is dependent on the oversampling ratio, the sigma-delta modulator order and quantizer resolution. Theoretically, doubling the oversampling ratio should result in a 3dB improvement in S/N, and increasing the order of the sigma-delta modulator by one should produce another 6dB improvement in S/N within the desired bandwidth. A 1-bit (two value) increase in quantizer resolution should yield another 6dB. As the sigma-delta converter order is increased, however, phase delay increases, leading to circuit instability. This means that increasing the order requires stiffer specifications for the phase compensation circuit.
To relax the design load for the phase compensation circuit, Asahi Kasei lowers the number of orders from the usual four to only two, and instead boosts overall S/N by increasing the quantizer resolution from the conventional two to four values. The oversampling ratio (Fig 3) is left untouched at 128x. According to the firm, they have another reason to lower the number of orders to two; three or more would make the circuit susceptible to oscillation due to phase delay. The company has not disclosed concrete circuit details.
Burr-Brown Japan has improved all three items: oversampling ratio, orders and resolution. The oversampling ratio is boosted from 8x to 64x, the 3-order sigma-delta modulator increased to a 4-order design, and the quantizer from a 5-level to an 8-level resolution. Overall S/N, as a result, is improved from 96dB for the prior design to 106dB. As the number of orders is increased, however, output non-linearity is also easily increased, and the firm stabilized operation with a newly designed compensation filter.
Segment-Type DAC Attains 120dB
Burr-Brown Japan is also developing a segment-type 24-bit DAC (Fig 4). "The S/N ratio is about 120dB; the best on the market," says Hajime Kawai, application engineering manager at the firm's E/EP Division. Dynamic range is also an excellent 112dB. The chip uses sign magnitude coding, so that two data with equivalent absolute values are encoded as the same codes, processing positive and negative data in two separate DACs. The DAC IC output is the total of the outputs from these two internal DACs. In principle, this means there is no zero-point error. The objective is to minimize conversion error for minute signals.
The cost of a DAC IC achieving n-bit resolution with 2n-1 constant current sources is higher than that of a sigma-delta DAC, because built-in resistors and DAC gain must be trimmed with a laser machine. Some engineers in the field claim that this alone can boost costs by ¥1,000, making the total chip as much as five times more than a sigma-delta DAC.
Segment-type DACs require off-chip current-voltage conversion circuits and analog low-pass filters. To assure the S/N and dynamic range cited in the catalogs, the temperature and other characteristics of the required operational amplifiers and resistors are also stringent. Because the performance of the current-voltage converter and low-pass filter affect the overall AV system S/N, the load on the designer increases.
Aiming at Prices under ¥200
Crystal Semiconductor is focused on cutting the cost of the chip, and plans to ship the CS4334 in March 1998 with an S/N of only 96dB, but a unit price of only ¥200 in lots of 1,000. Noritoshi Noso, field application engineer, Crystal Products Marketing, Cirrus Logic, comments, "We developed the chip for applications which don't require such high S/N ratios, like game systems and automotive equipment using DVD-ROM or DVD playback functions. We will be able to drop the price below ¥200, depending on quantity."
The chip will be manufactured using 0.35µm rule complementary metal-oxide semiconductor (CMOS) technology. This will yield a small chip size and a 225-mil 8-lead small outline package (SOP). "The package has the same lead count and lead width as the previous 20-bit model," continues Cirrus' Noso, "It can be swapped into existing sets immediately, and its small footprint is a major advantage over competing products."
Supply Voltage Lowered to +2.7V
Philips and Asahi Kasei Microsystems are developing products that improve S/N while at the same time lowering the supply voltage for the 24-bit DAC from +5V. Philips will ship the UDA1328 in April 1998. It will support a supply voltage range from +2.7 to +3.6V, and Asahi plans to develop an IC running at +3.3V.
"We wanted our product to be used in +2.7V mobile equipment," explains Yasuhiko Sugisawa, assistant group leader, IC Design Center, IC Development Department, Philips Japan, Ltd.
When the supply voltage is dropped from +5 to +2.7V, however, the output signal also drops proportionally, and would normally result in a lowered S/N. Compared to a 5V chip, the difference in S/N would be about 5dB (half) for a 2.7V chip.
The UDA1328 uses a sigma-delta converter with four orders instead of the usual three, which assures an S/N of 103dB about 4dB better than the predecessor chip.
6-Channel Output Designs
Philips, in addition to lowering the supply voltage to +2.7V, also offers a 6-channel output. Burr-Brown Japan plans to develop a 6-channel chip as well. Until now, DAC ICs have generally only offered one or two channels.
The 6-channel output provides support for the 5.1-channel Dolby Digital standard (formerly called Dolby AC-3). With a single chip outputting 6 channels, the
required IC footprint is reduced; with 2-channel DACs, three chips were required. The 6-channel chips, however, are more susceptible to interference between the output circuits, which would lower S/N. Philips solved this problem with an undisclosed proprietary design approach.
by Hui Cao
