1-bit DAC

A 1-bit DAC (sometimes called Bitstream converter by Philips) is a consumer electronics marketing term describing an oversampling digital-to-analog converter (DAC) that uses a digital noise shaping delta-sigma modulator operating at many multiples of the sampling frequency that outputs to an actual 1-bit DAC (which could be fully differential to minimize crosstalk).^[1] The combination can have high signal-to-noise and hence an equivalent effective number of bits as a DAC with a larger number of bits (usually 16-20).

The advantages of this type of converter are high linearity combined with low cost, owed to the fact that most of the processing takes place in the digital ___domain, which helps relax the requirements for the subsequent analog low-pass filter (for anti-aliasing image frequencies and suppressing high-frequency noise-shaping noise). For these reasons, this design is very popular in digital consumer electronics (CD/DVD players, set-top boxes and the like).^[2]

While single-bit delta-sigma DACs have an advantage of a much simpler internal DAC, multi-bit delta-sigma DACs have the advantages of a simpler digital noise-shaping loop, less dithering, a much simpler analog smoothing filter, and less sensitivity to clock jitter, so generally the advantages of multi-bit truncation outweigh single-bit truncation.^[3]

Linearity

Multi-bit DACs may have severe linearity error, mainly due to mismatch in components, with significant error when the MSB changes (called "zero-crossing distortion"). But a 1-bit DAC only has a gain error, since its transfer function has only full-scale positive and negative reference points, with the intermediate points determined by time averaging. While multi-bit DACs try to seek resolution in amplitude, 1-bit DACs instead compensate by taking advantage of digital filtering combined with the high accuracy in time provided by quartz crystals.^[4][5]

Oversampling Examples

For example, the 1991 Philips SAA7322/3 ^[6] first upsamples 44.1 kHz 16-bit input audio by 4x with a low-pass FIR filter to produce a 176.4 kHz intermediate signal, which again upsamples 32x more while adding a 352 kHz digital dither signal, and then upsamples 2x more to produce a 11.2896 MHz 1-bit output (a total of 256x oversampling).

Another 1991 Philips example, the SAA7350 ^[7] oversamples a total of 384x, which for its maximum input frequency of 53 kHz 20-bit audio results in a 20.35 MHz 1-bit output.

"Bitstream"

Philips also used the "Bitstream converter" trademark for the TDA1305 ^[8] which is also an upsampling converter (by 384x), but which ultimately outputs through a 5-bit DAC. So it is not clear that "bitstream" by Philips necessarily refers to a 1-bit output, but could include other DACs that utilize large upsampling into low-bit DACs.

The word "bitstream" is also a general computing word for a sequence of bits, used in many contexts other than 1-bit DACs.

See also

• Digital-to-analog converter § Types
• Direct Stream Digital

References

1. Sneep, J.; Kup, B.; Pelser, F.; Dijkmans, K. (1991-05-07). "A bitstream D/A Converter for Top Grade Digital Audio". 1991 IEEE International Conference on Consumer Electronics. pp. 294–295. doi:10.1109/ICCE.1991.733197. ISBN 0-7803-0001-7. S2CID 61847063 – via IEEE Xplore.
2. Smith, Steven W., The Scientist’s and Engineer’s Guide to Digital Signal Processing (Second Edition). San Diego CA: California Technical Publishing, 1999 (ISBN 0-9660176-6-8), p. 60.
3. Pavan, Shanthi; Schreier, Richard; Temes, Gabor C. (2017), "Delta-Sigma DACs", Understanding Delta-Sigma Data Converters, IEEE, pp. 425–450, doi:10.1002/9781119258308.ch13, ISBN 978-1-119-25828-5, retrieved 2023-06-30
4. Shah, Prasanna (1991-01-01). "Audio Magazine (Jan. 1991): Music of the Bitstream" (PDF). www.gammaelectronics.xyz. pp. 56–64. Archived (PDF) from the original on 2023-03-30. Retrieved 2023-06-29.
5. "The Philips Bitstream method: An introduction". DutchAudioClassics.nl. Archived from the original on 2023-03-02. Retrieved 2023-06-30.
6. "SAA7322/7323 Data Sheet" (PDF). 2025-08-13.
7. "SAA7350 Preliminary Specification" (PDF). Retrieved 2025-08-13.
8. "TDA1305T Data Sheet" (PDF). Retrieved 2025-08-13.