CD přehrávače s možností vyššího převzorkování

CD, SACD, DVD přehrávače, DAC převodníky, hudební servery, PC atd.
Renown
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Re: CD přehrávače s možností vyššího převzorkování

Příspěvek od Renown »

DSD over PCM je dohodnutý způsob přenosu DSD signálu za použití PCM frames přes USB, ale lze to i přes S/PDIF.
Je to tak, že se v rámci 24 bitového PCM frame přenáší nejprve 8 bitů marker, které DACu říkají, že se nejedná o PCM, ale že následujících 16 bitů bude DSD signál. Takže v 24 bitových frames se přenáší 8 bitů markerů a 16 bitů DSD signálu. Datový tok tedy musí být o jednu třetinu vyšší, ale při dnešních rychlostech USB je to naprosto ok. Akorát to nemůže být jakkýkoliv DAC, ale musí to být DAC, který dokáže identifikovat DoP a zpracovat DSD.

tmwatch
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Re: CD přehrávače s možností vyššího převzorkování

Příspěvek od tmwatch »

To Renown: díky moc. Můžeš mi prosím tě napsat příklad použití DoP, neumím si to představit. Pokud je podmínkou DAC, který umí DoP a zpracovat DSD signál (např. Soubor s příponou DSF), proč mu jej neposlat přímo v tomto formátu? Pokud rozumím správně, když DAC umí jen PCM tak signál DSF "zabalený v PCM obálce" nepřehraje. Je v tom nějaká výhoda?

Renown
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Re: CD přehrávače s možností vyššího převzorkování

Příspěvek od Renown »

1. Motivation :
The USB Audio specification 2.0 defines multiple formats for audio, of which standard PCM is only one. A general "raw data" format was also defined that can be used for any kind of data including audio, but unfortunately, no specific format was defined for DSD. With the ongoing proliferation of USB converters in the current market, it appears that the opportunity for the official USB specification to adopt a single common method of transferring DSD audio via USB is slowly disappearing. This article is an attempt at uniting as many manufacturers as possible and jointly defining a method for transferring DSD via USB.
The manufacturers developing audio playback software want to minimize the number of formats they need to support for a USB link. Ideally there is only a single such format. Likewise hardware manufacturers want to make their hardware compatible with as many playback platforms as possible. That, of course, only happens when all use the same format.
As mentioned above, USB Audio already supports a "raw data" format that could be used for DSD and that would create a clear separation to any audio data path containing PCM. However, the latest release of Apple’s operating system OS 10.7 incorporates a USB driver that only supports PCM. Furthermore, the central audio engine, CoreAudio, inside the OS only supports PCM as well, though luckily with no limitation on sample rate. (Earlier versions of the Apple OS supported a mode that was compatible with raw data mode, but that is history now.) Since the architecture of Apple’s OS forces audio software developers to use CoreAudio for everything audio related, there is basically only 1 format left for the Mac platform: PCM. Creating a separate path for DSD would involve a lot of surgery, if it is even possible. So we have no choice but to use a PCM path to transfer DSD audio, by using special flags or headers that allow the receiving hardware to detect a format change and switch their decoder accordingly.
When using the Windows platform things are little easier: Windows by nature does not fully support USB Audio 2.0, and what it does support is limited to PCM only at a sample rate of 96kHz or less. There is no native driver support for higher resolution PCM, and it is clear from the beginning that a custom driver needs to be created for this platform, whether it is for regular PCM or DSD. Luckily a 3rd party software developer (Steinberg Audio) jumped in and created a driver (called ASIO) already many years ago that supports PCM and DSD, with no limitation on sample rate or wordlength. It has become quite popular, and many software vendors support this in the meantime. ASIO is not directly a hardware driver, but sits between the audio playback application and the hardware driver. Each hardware manufacturer still needs to develop a custom hardware driver for their own hardware, but ASIO then creates a common interface standard for all application software.
2. Solutions
As seen above, the Windows platform basically offers a solution with the ASIO driver and the raw data format supported by USB Audio 2.0. This is not as ideal as having a dedicated DSD path via USB, but this is safe and straightforward.
Since the Apple OS only allows a PCM path, we have to find a way to put DSD audio data into PCM frames, which are then sent via the native USB driver. DSD has a sample size of 1 bit and a sample rate of 2.8224MHz. In other words, the data rate is 2.8224Mbits/sec. This is equivalent to 16-bit PCM at a rate of 176.4kHz. In order to clearly identify when this PCM stream contains DSD and when it contains PCM, we will need additional bits. The PCM format with the next higher bit rate is 24 bits at a sample rate of 176.4kHz. This gives us 8 extra bits for this marker of identifier. It seems like a bit overkill if all we need is 2 states (8 bits give us 256 states), but we will see that this extra overhead comes in handy. Here is how we can use the 24 bits in each sample and for each channel:

The 8 most significant bits are used for the DSD marker, and alternate with each sample between 0x05 and 0xFA. Each channel within a sample contains the same marker. This has been chosen to minimize the click that might be experienced when the receiving hardware misinterprets the data as PCM when it really is DSD. If this should happen, it would create a tone around 88kHz and roughly -34dB, nothing harmful and something that most D/A converters would suppress to some degree before it even reaches the loudspeaker. It should be pointed out that hardware manufacturers and software developers alike can easily use common safeguards to prevent such cases of erroneous format switching, and that they may only be limited to times during development of hardware and software. It is their responsibility to prevent misinterpreted cases and to test their products thoroughly before release. Misinterpretation of PCM data as DSD may create less predictable clicks.
The remaining 16 lower bits are then used for the DSD data, first or oldest bit in slot t0. The USB Audio specification assigns each PCM Frame to a specific channel (left, right, etc.), and when used for DSD streaming, each PCM Frame contains only DSD data corresponding to its assigned channel.
How to set up Foobar for DSD playback can be found on Project Audio Systems website.

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