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Why does USB feature so much in discussions about DACs when the newer HDMI seems better?

I am a bit confused about the frequent mention of USB in the context of stand-alone Digital to Analog Converters (DAC).  Why is HDMI left out?  Is this a US versus Europe / Asia thing?

The Universal Serial Bus (USB) was introduced in 1996 by a group of computer manufacturers primarily to support plug-and-play for peripherals like keyboards and printers.  It has only two signal wires, plus two wires that can supply DC power.

The High-Definition Multimedia Interface (HDMI) was specifically designed by a group of television manufacturers for transmitting digital audio and video in many formats.  It hit the shops around 2004.  There are 19 pins supporting four shielded twisted pairs, and seven other wires (3 of which can instead form a shielded twisted pair for Ethernet).

I have three universal disk players from Sony, Panasonic and Reavon, which all have two HDMI outputs, one can be dedicated to audio only, the other carries video or video plus audio.  (Only the Panasonic does not support SACD).  My Marantz AV 8802 pre-processor has 11 HDMI connections and only two USBs.

Of course, both USB and HDMI continue to evolve.  Then there is the Media-Oriented System Transport (MOST) bus designed by the automotive industry, which looks even better.

Why is it so?

richardbrand

Showing 1 response by zlone

zlone
829 posts
 

I am not disagreeing that HDMI might be superior in some ways, but I think the answer is that USB works. And from an engineering and cost standpoint it would be difficult to make the case for using HDMI. More wires does not mean better, USB is more than capable of providing the bandwidth for hi-res audio. 
 

As others have stated, HDMI connections have been popping up on DAC’s to accommodate I2S, not because it is inherently supported by the HDMI standard, but because I2S requires more wires to pass the signal. 
 

I will step out on a technical limb here, so please feel free to correct me, but USB audio uses a packet protocol similar to most digital communications systems. Whereas I2S is a low level hardware signaling protocol meant to be used on short buses between hardware components. Is it better? Very possible, but it is totally up to the implementation, which for now has no standard, and may be prone to corruption if not done well. USB, by virtue of using a packet protocol has some verification capabilities to ensure accurate data transmission, I don’t know, but I am guessing the I2S signaling does not have this capability. 
 

Back to the answer, they use USB because it works and makes technical and economic sense. Again, not disagreeing that I2S might sound better under the right circumstances, but if you ran the same packetized protocol on HDMI that you run on USB, there would be no inherent advantage, and it would be overkill. 

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