USB vs other A/V cables?

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DarkDubzs

DarkDubzs

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Jun 10, 2013
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Probably a dumb question, but just popped in my mind and cant get over it. Why can we transfer so many different types of data via USB? Like, we can transfer video, audio, and other bytes of data on pc's and similar hardware. Although, why can we not use USB cables for things like to watch tv or telecommunications, etc? Why is it that we need to buy and use specific cables like HDMI for audio+video, AV for video, DVI for video, VGA, coaxial, 3.5mm for audio, etc, when USB seemingly can transfer all that data? Im 99% sure there is an easy answer for this, i believe it would be something like because data to watch tv is different from data on a pc or something like that... hard to explain. I just love to question things, wonder, and computer hardware and networking, so, sorry for this small question. Please help me out with this question. Thanks!
 
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hunter315

hunter315

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May 12, 2009
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I'm going to work backwards on the list.

You use a 3.5mm audio jack because it carries an analog signal and has it split for the left and right channel so when it arrives at the end there is no processing that needs to be done. There are USB headsets which do receive their audio data over USB then convert it into the analog audio signal needed for the speakers.

Coaxial cable(specifically TV cable) actually carries an RF signal, it can carry tons of different ones that are at slightly different frequencies giving you dozens of different TV channels, the bandwidth of a television Coax cable is orders of magnitude higher than USB as it ships video with no compression whatsoever.

VGA carries analog video, if you were to use USB you would have to have processing at the other end to convert it into what the monitor needs, and since VGA was a standard prior to USB it gets to live on.

DVI and HDMI are both far superior to USB in terms of bandwidth, single link DVI has a bandwidth of near 4Gbps while USB 3.0 has an effective bandwidth of about 3.2Gbps but is more recently introduced than either of those connectors, USB 2 only has a transfer rate of ~480Mbps which is far to slow for live HD video. You would also begin to suffer problems with noise and dropped packets if you attempted to consistently feed video signal of that level through USB. USB was meant to be cheap, not super reliable, but if you start losing packets with a video feed you are going to start getting corrupted frames, DVI and HDMI are meant to be robust and allow you to do most of the processing in the computer rather than in the monitor.

Display port is the future connector for monitors and can give you a bandwidth of up to 17.28 Gbps.


Most of the cable choice comes from deciding where you want the encoding/decoding stage, what is popular at the time, and what will give you needed expansion for the future.
 
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InvalidError

InvalidError

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May 18, 2007
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Until integrated 100/1000Mbps Ethernet became standard in most PCs, there used to be xDSL and cable modems with USB ports but now that most people use broadband routers, those have gone nearly extinct.

You can buy USB-based headphones and DACs instead of using coax/2.5mm audio jacks.

There are also USB-based TV tuners, WiFi dongles, Bluetooth dongles, 3G/4G dongles, etc.

So, most of those things you are wondering why they aren't done over USB are available in USB form. The main reason those are not too popular is because they add significant cost, clutter and complexity: an HD-Audio chip on a motherboard costs $3 to implement and connects directly to your audio amplifier or headphones with no need for audio drivers beyond the generic HD Audio driver. For USB audio, you end up with the PCI driver being involved between the CPU and xHCI USB controller, the USB HID drivers to communicate with the USB DAC, optional additional drivers to control DAC features, the DAC chip, case and wiring for the external DAC, etc.

There are some external USB displays on the market but those are not very popular due to being little more than dumb frame buffers since USB does not have enough power budget or bandwidth to accommodate a strong backlight and remotely decent GPU. With USB 3.1 and the recently adopted 12V/20V high-power spec, we might see new USB displays with some GPU acceleration or possibly the ability to stream output from the system's discrete GPU or IGP over PCIe and USB3. At 5Gbps, USB3 is barely enough to stream raw 1080p60 and grossly insufficient to feed any modern desktop GPU.
 
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