I want to discuss some of the new formats that have been mentioned
before, but not yet implemented commercially or by amateurs (that I
know of, please tell me if I'm wrong).
Specifically, the formats I am referring to are:
- Multi-bit Delta Sigma, either 5 or 6 bit at 11.2892Mhz or 12.288Mhz (256fs) sample rates or higher, which would effectively be a "RAW" capture from today's best cutting edge low SNR ADC's (there are 1gs/s+ adc's with worse SNR = less effective bits).
- 1-bit DSD at 256fs and higher sample rates. Anecdotal reports mention that 256fs appears to be the first point in 1-bit audio where the noise and signal don't interfere, therefore removing the need for noise shaping.
- 768 Khz PCM. The highest bitrate PCM I've seen used, but at 16-bit, and only for scientific use. Not sure what ADC's support this, but the Arda AT1401 DAC supports up to 1.5Mhz PCM.
The reasoning behind the proposed format of 6-bit 12.288Mhz DSD, is
the same reasoning that created DSD. The highest quality ADC's of
today use a multiple order delta sigma converter, which is than
processed into either 1-bit Delta sigma, or multi-bit PCM,
theoretically loosing some information in either conversion. By
capturing the "raw" output of the ADC, all mathematical interpretation
can be done at a later point, allowing conversion to other 'future'
formats as they appear, or reinterpretation when new conversion
methods become standard.
It should allow capturing the maximum obtainable quality in archiving
priceless audio. All the disadvantages of DSD still hold true, lack of
ability to perform manipulations or process without conversion, but
now that we have moved past monolithic single-bit delta sigma ADC's,
the archival use of single-bit DSD is questionable. I believe the
"best" mathematical conversion is open to debate, as there is no
perfect solution. By capturing the "raw" output of a ADC allows the
highest amount of care in selecting which algorithm is best in down
conversion. It should allow the maximum quality to be obtained, and
allows choosing a possibly superior conversion at a later date should
another option arise.
I believe that this project has many applications beyond simply audio,
perhaps an open-source SEM (scanning electron microscope) would
benefit from a high speed interface for an ADC for capturing data for
processing, Hams could use it for extending the utility of SDR
(software defined radio's) to allow capturing/sending data, or a radio
telescope project could benefit from more precise digitization,
allowing linking up online and using aperture-synthesis to increase
the effective aperture. Ultrasound systems and other medical imaging
devices like MRI or CAT scans could be possibly utilize it. For
medical imaging lower starting costs are important for the developing
world. GPR (ground penetrating radar) could be developed for the
developing world to survey land quickly and cheaply, to find hidden
dangers or resources.
The uses are endless, as we are in a society where there is a never
ending need to capture, create, and analyze information. Open Hardware
and Software is a revolution, and everything entered into the public
domain can be reused and modified to find uses far beyond the original
intention. For this reason I hope to create some momentum behind
developing a General Purpose High speed ADC/DAC interface. Almost no
design details exist as of now, except the proposed format. This is a
similar project to the SDR-Widgit project, but with the expanded goal
of supporting the "RAW" output of modern Delta-Sigma ADC's. Possibly
SDR-Widgit or Audio-widgit will be used as a starting point in
development.
The need for a new archival and mastering format, as well as a format
for scientific research is becoming more needed every day. For studies
of psycho acoustics, echolocation, animals, sonar, ultrasound,
turbulence, aerodynamics, and virtual reality there is no lack of need
for increasingly precise digitization of audio. There are devices sold
today sampling at rates up to 768Khz for Bat studies. As new
technologies develop, in addition to computers becoming faster at an
exponential rate, the need for easy ways to capture data accurately
have never been greater.
I am hoping to start an open-source community that can help to develop
a high quality high speed digital interface. This project will (*most
likely) require an FPGA and custom programing and hardware to take the
high speed multi-bit or single-bit data and accurately capture or
output it, without conversion to other formats.