Synchronizing Separate Input (ADC / Analog to Digital Converter) and Output (DAC / Digital to Analog Converter) devices
From time to time, we receive a question from someone considering using our Pure Vinyl software regarding choosing an analog to digital converter (ADC) for audio input, because they only have a separate, stand-alone DAC, concluding that the best solution is a stand alone ADC. Why duplicate functionality?
That's a logical chain of thought. However, "adding" a stand-alone ADC to an existing, stand-alone DAC is an impractical means of incorporating ADC functionality into a digital audio system.
Aside from the issue of having multiple separate audio devices connected, which may cause unexpected behavior (it's best with USB or Firewire audio devices to only have one active connected audio device at a time), there is a fundamental issue with stand alone ADCs (that is, a product with ADC functionality only, instead of having the ADC and DAC functions in the same product). The issue occurs because (in the absence of special hardware features) a stand alone ADC cannot synchronize its audio stream / clock with a separate DAC.
When using different audio devices for input and monitoring during recording of an LP, e.g., separate ADC for input and DAC for output, the word clocks cannot be synchronized. The result is that you will get "clicks" or noise in the monitored audio.
- If you record an LP and then play it back, there is no problem.
- However, using different audio devices for the ADC and DAC for monitoring live LP playback, or monitoring during recording will probably not work, and you will hear clicks in the "live" audio.
It would be the same as having a car with two independent engines, one driving the front wheels and the other driving the back wheels: no way to perfectly synchronize the drive trains. Same thing here, except the "drive trains" are the separate sample clocks in each audio device.
This is a well-known issue in the field of digital audio.
Audio devices found in recording studios (when stand alone ADCs and DACs are employed) have "word clock" (WC) synchronization connections, so that all devices can be synchronized to the same sample clock. However, word clock ports are usually not provided on audiophile ADC gear. But that's not all: for ADCs that may have a WC port, this is useless unless the DAC also has a corresponding WC port of the proper "gender" e.g., WC "out" on the ADC requires a WC "in" on the DAC, or vice versa. This also increases the complexity of the setup, and with modern low-jitter clocks that now are built in to audio devices, connecting an external clock actually decreases reliability and increases undesirable clock timing jitter (reducing the audio quality).
Consequently, it has become extremely unusual (in the pro-audio market) to find stand-alone ADCs. No pro-audio hardware manufacturer has introduced a stand-alone ADC for many years. This is not a marketing decision; pro-audio profit margins are thin, and if anything, the incentive would be to omit the DAC functionality for cost savings - so one would expect to find a preponderance of stand-alone ADCs. The opposite is true. In the professional and home-recording markets all new analog to digital converter products introduced in the last several (6 +) years, as a rule, incorporate ADC and DAC functionality in the same box. Many audiophile-oriented hardware manufacturers haven't come to this realization and have introduced stand alone ADCs (as a first foray into creating an ADC product), with no way to synchronize a DAC. For that reason alone, we cannot recommend any such stand-alone ADCs. (Examples of such stand-alone ADCs include the PS Audio NuWave Phono and the M2Tech Joplin.)
Back when we began creating our audio software products well over 14 years ago we expended a lot of effort on trying to get multiple separate audio devices to work together satisfactorily, and it was fruitless. Additionally, the Apple OS provides a way to combine multiple devices into a single, "virtual" device (called an "Aggregate Device") but this will only work reliably if the sample clocks are already synchronized (via external word clock connections).
The only reliable and extremely easy way that is guaranteed to allow artifact-free audio monitoring is with a device that has an ADC and DAC in the same box. There are dozens and dozens of such products to choose from, covering a wide range of prices from entry level to high-end.
If you have a favorite DAC that you prefer to use to play the recordings, rather than the DAC in the audio recording device, no problem. The issue only arises during real-time (live) monitoring of the audio, where both the ADC and DAC are in use, and only on the monitored audio (not in the recordings).
Return to Getting Started with Computer Audio page