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acuvox

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  1. Eloise, you have mis-understood every issue. 1) Jitter is always a problem. It is atrocious in PC/Mac environment. Hard Disk storage latency and seek times are milliseconds and acceptable jitter is nanoseconds, so you are off by a factor of 1,000,000. There are typically two layers of cache and several RAM buffers between HD and ADC and those allocations are not large in terms of hi rez audio. The processor could keep a RAM buffer full but it is constantly being interrupted by megabyte virtual memory swaps and translating images to the screen, so it doesn't always transmit at the right time. USB Drivers have low priority unless you invoke "isochronous mode" which is usually called asynchronous. Even then, you need to have one port dedicated to audio. The computer also generates a generous amount of RFI which couples down the USB cable to the interface and induces jitter, as well as motors, dimmers, cell phones (even when you are not talking), microwaves, TVs, fluorescent lighting and other computers in the building. Vibration can also cause jitter because quartz oscillators are mechanical devices. 2) I don't know of ANY pro audio or video gear made by computer companies. The business models are worlds apart in terms of manufacturing volume. Pro audio devices have drivers that replace the OS handling of audio and USB to obtain sufficient performance, but that brings up compatibility problems since they now have to deal with every variation of "standardized" hardware. This is one reason the pro AV market is slanted towards the more consistent environment in Macs - but they still need live customer support for nearly every pro AV installation. A lot of pros don't use computers at all, relying on dedicated digital consoles. 3) Isochronous is not applicable to pro audio because it does not support multi-track. The term "Asynchronous USB" refers to a provision of the USB Audio Class 1.0 standard, which AFAIK has not yet been officially updated. USB 2.0 still uses the USB AUDIO CLASS 1.0 spec, please do not confuse these two even though the terminology is obviously confusing. USB Audio Class 2.0 supports 24/192 and is in beta testing. Check this out: http://www.amr-audio.co.uk/html/dp777_individual.html 4) Since there is no native FireWire support in Windows, all FireWire interfaces require drivers. You can write that driver to support whichever protocol. Audio and video devices use a protocol for streaming, that is regular packet flow where underflow is not a problem. In lieu of overflow or underflow and given a stable oscillator, phase locked loops can be built with low jitter even if they are multi-frequency - it just increases power supply cost and settling times. In the pro world you can get source material with clocks a bit off so PLL receiver makes more sense.
  2. "asynchronous" is specific jargon in the USB world that really means isochronous - that is, the receiving device maintains streaming rate by stable local clock and gating packets from the transmitter to prevent buffer overflow and/or underflow. True asynch as in RS-232, RS-485 have no underflow or "request to receive" signals. In the pro world you always want to synchronize multiple boxes, for movie and video SMPTE synch is necessary. Pro gear typically has clock in and out and master clocks will cost thousands for an oven stabilized reference, very sophisticated PLL clock recovery from incoming sources and fast clean output buffers. (In pro music, usually from a company starting with A as in Aardvark, Antelope, Apogee). IEEE1394, commonly known as "FireWire", is designed as a streaming protocol which is why it is preferred for high performance audio and video; and by high performance I mean for example 720p/60 or 24 bi-directional channels at 24/96, two dozen times standardized USB "high resolution" audio. Ethernet pumps the signalling rate so high (100Mb-1Gb) that it can still use Collision Detection/Resend for consumer audio, and is inherently asynch. In this case, asynch is NOT a good thing. Ethernet sucks for pro audio data, it is typically only used for control or archiving in burst transmission. There are more affordable QoS and static routing switchgear in the market now but you still need to segregate hi-rez streaming data in a subnet. Pro USB gear historically used high performance drivers to replace all the Windows or MacOS junk. The computer then became dedicated to audio production and typically YouTube videos, iTunes or Windows Media Player would not come out of the interface at all. Just as well to dump all the stupid OS sounds, which blast your studio monitors and ears. Pro users optimize their production machines, i.e. don't connect them to the internet, don't run browsers nor email nor Office software on them and clean out the registry. S/PDIF is synchronous clocked by the transmitter - which is fine if it has absolute priority over virtual memory disk swaps, housekeeping and other OS routines OR runs on a dedicated PCI card. In multi-core machines you dedicate a processor, prioritize the interrupt and everything can be peachy. Problem is, this is a lot of non-standard configuration for a tiny niche market. PCs of either flavor are optimized for the LCD user. (and I'm not talking liquid crystals). This is why Mr. Rankin is currently the guru of high-end USB, with his excellent business model of licensing drivers to cover market price points.
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