I'm absolutely not an expert on this but my understanding is that BNC is significantly better than coax from a jitter perspective.
18 responses Add your response
I suspect that you meant to ask whether a BNC connector or an RCA connector is preferred. The answer would be the BNC connector -- it is designed to handle high frequencies, and will provide a better impedance match to the cable (which is important for relatively high speed digital signals such as SPDIF, although unimportant for analog audio signals).
Thank you all for your replies.
I do understand that BNC is a connector, not a cable.
My understansing is that HD video signals (like from DVD and BD) are faster (greater bandwith?) than digital audio signals (from CD, PC). I have heard that BNC beats coax for video (via composite analog or by 75ohm digital), but is about the same for digital audio. Comments?
I have heard that BNC beats coax for video
Once again, BNC is a connector, and coax is a cable, so your question is unanswerable. Assuming you mean to ask whether a BNC connector is better to use for both video and digital audio than an RCA connector, the answer is yes (although either connector could very well be acceptable in both cases).
As Almarg says, coax is a cable. The name is a short form of "co-axial", which means that the central conductor and the woven shield share the same axis in cross-section.
Also as he says, you can fit an RCA connector or a BNC connector to a coaxial cable.
You could also fit twin-lead cable to an RCA connector but IME it would be a bit harder to do that with a BNC.
Other connector types can also be used with coax cable. Phone plugs for example.
SPDIF (Sony Philips Digital InterFace if I recall) is the protocol transmitted across the cable, and is used whether the coaxial cable is terminated by RCA or BNC connectors.
The SPDIF protocol includes left and right channel audio data, flags (e.g. to indicate pre-emphasis), subcode information (e.g. track and disk/track time info), and clock.
Here is more information on the SPDIF protocol, which as Ghostrider45 indicates can be transmitted over coaxial cable that can be terminated with either RCA or BNC connectors (and can also be transmitted over optical cable):
I wanted to add, in case it proves to be relevant, that RCA-to-BNC and BNC-to-RCA adapters are readily and cheaply available, although of course it is always good practice to avoid adapters where possible:
So, is the concensus that RCA termination or BNC termination is equally reliable for transmission of digital audio (say between transport and DAC)??? Or are there quantifiable differences between the two type of connectors?
I don't think it's readily quantifiable. The degree of sensitivity to impedance mismatch would be dependent, among other things, on the risetimes and falltimes of the digital pulses, which are not specified and probably differ among different components; the length of the cable; the amount of mismatch at both ends; and other factors. And the differences that the connectors make (such as the effects that impedance mismatch might have on jitter, resulting from its effects on waveform quality) might be overshadowed by other differences between the components being compared.
The best guide is probably just reading about other people's experiences with the particular components, and if possible auditioning them yourself.
Why would you choose the BNC connector over RCA? Tighter connection? But, there are "locking" RCA connectors.
It seems that cables w/ BNC connectors are always more expensive than the same cable with RCA connectors. Also, with many components RCA connections come standard, but it costs extra for BNC connections.
Is it possible that BNC connections/connectors are a carry over from pro audio where a secure connection is vitally important.
Tobias - I'm not sure how true 75 Ohm RCA plug could be but I used Canare 75 Ohm RCA plugs and Canare wire to build my cable. Severity of impedance mismatch, as Almarg explained, depends on rise/fall times and distance. It is common for CD player to have RCA output and for DAC to have BNC input. In such case you have to have either RCA plug or adapter. Get one that is 75 Ohm.
Characteristic impedance at high frequencies depends on ratio of inductance to capacitance (square root to be exact) what translates to geometry of the cable. Same is for RCA plug but dimensions here are already defined and probably not making 75 Ohm. Rest of the plug (body and pin to solder to) might follow 75 Ohm geometry.
Typical CD player has about 25ns rise/fall time. If it's to slow then noise will cause jitter (affecting edges) and if it's too fast (like 5ns) than you'll get reflections on impedance boundaries. Either way it will translate to jitter. My DAC (Benchmark DAC1) is jitter suppressing (upsampling) so cable and transport is not very important to me. Jitter increases noise.