RCA to BNC - any point?

Hi all,

I have a Codec (2ADCs and 2DACs), which is capable of 16bit per
channel sampling, at 96kHz. It interfaces to a DSP board. I have
designing a pcb board capable of processing an analog music signal,
and I'd like to interface it to an ADC on my codec. I'd also like to
send a signal from the DAC to a second pcb board. I'm just wondering
what would be the best way of doing this? The Codec has phono RCA
connections. The line input on the Codec has a Signal to noise ratio
of 90dBs VRms. (for a 1k full scall input), but this does not include
the PCB RCA jack into which any input or output signal most pass. The
Codec is called the TLV320AIC23EVM by TI.

I hope to encase my pcb music boards, the codec dsp processor and
non-switching power supply in a box, as a final product - so I dont
need long cabling between the pcbs and codec.

What's the best way of preserving my signal to noise ratio? I was
thinking a RCA to BNC adaptor, then a short BNC cable to each pcb
(each with a BNC input/output connector).

Thanks for your help,

Barry.

Barry wrote:
> 
> I have a Codec (2ADCs and 2DACs), which is capable of 16bit per
> channel sampling, at 96kHz.
> The line input on the Codec has a Signal to noise ratio
> of 90dBs VRms. 
> What's the best way of preserving my signal to noise ratio?

The 90dB is not really high SNR so preserving it should not be a
problem. Just the reasonable PCB layout with the correct grounds and
return paths is enough. 

> I was
> thinking a RCA to BNC adaptor, then a short BNC cable to each pcb
> (each with a BNC input/output connector).

Don't worry about the connectors.
 
Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Barry wrote:
|| Hi all,
||
|| I have a Codec (2ADCs and 2DACs), which is capable of 16bit per
|| channel sampling, at 96kHz. It interfaces to a DSP board. I have
|| designing a pcb board capable of processing an analog music signal,
|| and I'd like to interface it to an ADC on my codec. I'd also like to
|| send a signal from the DAC to a second pcb board. I'm just wondering
|| what would be the best way of doing this? The Codec has phono RCA
|| connections. The line input on the Codec has a Signal to noise ratio
|| of 90dBs VRms. (for a 1k full scall input), but this does not include
|| the PCB RCA jack into which any input or output signal most pass. The
|| Codec is called the TLV320AIC23EVM by TI.
||
|| I hope to encase my pcb music boards, the codec dsp processor and
|| non-switching power supply in a box, as a final product - so I dont
|| need long cabling between the pcbs and codec.
||
|| What's the best way of preserving my signal to noise ratio? I was
|| thinking a RCA to BNC adaptor, then a short BNC cable to each pcb
|| (each with a BNC input/output connector).
||
|| Thanks for your help,
||
|| Barry.

That is really overkill, you can come away with a normal boardtrace. Look at
this flimpsy analog audio cable from CDrom to soundcard. Even inside the
computer with certainly much worse condition than your box it can preserve
far more dynamic than your  codec. If you really want quality use the Cirrus
http://www.cirrus.com/en/products/pro/detail/P8.html
this part is classes better than TI products.

ciao Ban

bg_ie@yahoo.com (Barry) wrote in message news:<731cea69.0311180643.7a93730c@posting.google.com>...
> Hi all,
> 
> I have a Codec (2ADCs and 2DACs), which is capable of 16bit per
> channel sampling, at 96kHz. It interfaces to a DSP board. I have
> designing a pcb board capable of processing an analog music signal,
> and I'd like to interface it to an ADC on my codec. I'd also like to
> send a signal from the DAC to a second pcb board. I'm just wondering
> what would be the best way of doing this? The Codec has phono RCA
> connections. The line input on the Codec has a Signal to noise ratio
> of 90dBs VRms. (for a 1k full scall input), but this does not include
> the PCB RCA jack into which any input or output signal most pass. The
> Codec is called the TLV320AIC23EVM by TI.
> 
> I hope to encase my pcb music boards, the codec dsp processor and
> non-switching power supply in a box, as a final product - so I dont
> need long cabling between the pcbs and codec.
> 
> What's the best way of preserving my signal to noise ratio? I was
> thinking a RCA to BNC adaptor, then a short BNC cable to each pcb
> (each with a BNC input/output connector).
> 
> Thanks for your help,
> 
> Barry.


Avoid the really cheap and nasty RCA connectors that can corrode
badly. They can actually act like diodes, causing severe distortion
(think of the old copper oxide rectifiers). Apart from that, your
application is pretty undemanding, and most reasonable connectors will
do. Its only when you deal with very small signals, such as microphone
outputs, that great care is needed. In this case it isn't RCA vs BNC
you need to worry about. It is the actual contact material. You have
no wetting current, so you need exeedingly oxide free contacts made
from expensive alloys that make gold look el cheapo :-)

Regards,
Steve

Thanks for your replies.

Just to recap. I'm building an instument which I wish to encapsulate
in a box with a user interface. Inside this box I will have a DSP
board and a daughter board codec, plus two pcb boards. The codec will
interface via its ADC and DAC to two pcb boards which I have designed
myslef. These PCB boards are part of the instrument and will be
included in the box. The Codec has RCA phono connectors, and I cant
see anything in the documentation to suggest they are anything but
regular - there certainly not gold plated anyway.

I obviously need to connect the codec to each of my two pcb boards
with a short length of cable. A phono RCA connector carries two
signals, a ground signal and the "signal" signal, as does a BNC
connector. But I'm familiar with the type of BNC cables which you
would find in any electorics lab. These cables are co-axial - they use
the idea of a faraday cage to help eliminate noise. But any RCA cables
I've seen simly involve two parralel pieces of wire, there I was under
the inpression that a short BNC cable would be a good inclusion in my
circuit. In otherwords, I could use an RCA to BNC to convert the codec
female ins and outs to BNC female, I could have BNC in and out
connectors on my pcbs, and connect using a typical bnc cable. Perhaps
RCA cables come in a co-axial design, and that would be the obvious
choice.

Thanks,

Barry.

?ine Canby wrote:

> Thanks for your replies.
> 
> Just to recap. I'm building an instument which I wish to encapsulate
> in a box with a user interface. Inside this box I will have a DSP
> board and a daughter board codec, plus two pcb boards. The codec will
> interface via its ADC and DAC to two pcb boards which I have designed
> myslef. These PCB boards are part of the instrument and will be
> included in the box. The Codec has RCA phono connectors, and I cant
> see anything in the documentation to suggest they are anything but
> regular - there certainly not gold plated anyway.
> 
> I obviously need to connect the codec to each of my two pcb boards
> with a short length of cable. A phono RCA connector carries two
> signals, a ground signal and the "signal" signal, as does a BNC
> connector. But I'm familiar with the type of BNC cables which you
> would find in any electorics lab. These cables are co-axial - they use
> the idea of a faraday cage to help eliminate noise. But any RCA cables
> I've seen simly involve two parralel pieces of wire, there I was under
> the inpression that a short BNC cable would be a good inclusion in my
> circuit. In otherwords, I could use an RCA to BNC to convert the codec
> female ins and outs to BNC female, I could have BNC in and out
> connectors on my pcbs, and connect using a typical bnc cable. Perhaps
> RCA cables come in a co-axial design, and that would be the obvious
> choice.
> 
> Thanks,
> 
> Barry.

There's nothing to keep one from using RCA connectors on shielded cable.
In fact, they were designed for that, as their radially symmetric design
hints. (They are, after all, also known as phono connectors, and
phonograph cartridges deliver very low-level audio signals that need to
be protected from hum.) All audio signal cables with RCA connectors are
shielded. There may be doubt about some speaker cables, but most of
those are co-axial too (Except home made).

BNC connectors are useful for connections that are moved often, but that
isn't their reason for being. They provide impedance continuity for
50-ohm coaxial cable, thereby minimizing reflections on the line. That
can be important for some uses in the MHz range and up, but it's
irrelevant for audio. Nickle-plated RCA connectors are as good as
perfect for audio in a non-corrosive atmosphere. Gold-plated connectors
are better for corrosive atmospheres and mandatory for those who also
insist on gold-plated buttons and flatware.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Jerry Avins wrote:

(snip regarding connectors for coaxial cable)

> There's nothing to keep one from using RCA connectors on shielded cable.
> In fact, they were designed for that, as their radially symmetric design
> hints. (They are, after all, also known as phono connectors, and
> phonograph cartridges deliver very low-level audio signals that need to
> be protected from hum.) All audio signal cables with RCA connectors are
> shielded. There may be doubt about some speaker cables, but most of
> those are co-axial too (Except home made).

I think the connectors are cheap to make in a production line.

> BNC connectors are useful for connections that are moved often, but that
> isn't their reason for being. They provide impedance continuity for
> 50-ohm coaxial cable, thereby minimizing reflections on the line. That
> can be important for some uses in the MHz range and up, but it's
> irrelevant for audio.

Well, in the GHz, or hundreds of MHz, unless the system is extremely 
sensitive to reflections.   RCA connectors are fine even in the low 
megahertz range, as commonly used for video signals.

> Nickle-plated RCA connectors are as good as
> perfect for audio in a non-corrosive atmosphere. Gold-plated connectors
> are better for corrosive atmospheres and mandatory for those who also
> insist on gold-plated buttons and flatware.

BNC connectors also have a more reliable spring to keep the contact 
tight.   If you squeeze RCA connectors once in a while that helps keep 
them tight.   Well, maybe it is more that RCA connectors rely on the 
friction between the connector tabs and the jack to hold them in place, 
in addition to keeping electrical contact.  They aren't always made of 
metal that is springy enough to do that well.  BNC connectors separate 
the contact function from the hold-in-place function, which can be 
useful even where impedance matching isn't important.

-- glen

?ine Canby wrote:

> Thanks for your replies.

(snip)

> I obviously need to connect the codec to each of my two pcb boards
> with a short length of cable. A phono RCA connector carries two
> signals, a ground signal and the "signal" signal, as does a BNC
> connector. But I'm familiar with the type of BNC cables which you
> would find in any electorics lab. These cables are co-axial - they use
> the idea of a faraday cage to help eliminate noise. But any RCA cables
> I've seen simly involve two parralel pieces of wire, there I was under
> the inpression that a short BNC cable would be a good inclusion in my
> circuit. In otherwords, I could use an RCA to BNC to convert the codec
> female ins and outs to BNC female, I could have BNC in and out
> connectors on my pcbs, and connect using a typical bnc cable. Perhaps
> RCA cables come in a co-axial design, and that would be the obvious
> choice.

I am pretty sure that the cables with RCA connectors that you commonly 
use with audio equipment are coaxial.  Well, I used to solder my own, so 
I know those were.  Though many stereo cables have two coaxial cables 
side by side, and look similar to other parallel wire cables.    The 
ones I used to make were with RG58 cable, but most commercial ones 
aren't that good.  (The ones that come free with audio equipment.)

-- glen

glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote in news:khNyb.275555
$ao4.946308@attbi_s51:

> ?ine Canby wrote:
> 
>> Thanks for your replies.
> 
> (snip)
> 
>> I obviously need to connect the codec to each of my two pcb boards
>> with a short length of cable. A phono RCA connector carries two
>> signals, a ground signal and the "signal" signal, as does a BNC
>> connector. But I'm familiar with the type of BNC cables which you
>> would find in any electorics lab. These cables are co-axial - they use
>> the idea of a faraday cage to help eliminate noise. But any RCA cables
>> I've seen simly involve two parralel pieces of wire, there I was under
>> the inpression that a short BNC cable would be a good inclusion in my
>> circuit. In otherwords, I could use an RCA to BNC to convert the codec
>> female ins and outs to BNC female, I could have BNC in and out
>> connectors on my pcbs, and connect using a typical bnc cable. Perhaps
>> RCA cables come in a co-axial design, and that would be the obvious
>> choice.
> 
> I am pretty sure that the cables with RCA connectors that you commonly 
> use with audio equipment are coaxial.  Well, I used to solder my own, 
so 
> I know those were.  Though many stereo cables have two coaxial cables 
> side by side, and look similar to other parallel wire cables.    The 
> ones I used to make were with RG58 cable, but most commercial ones 
> aren't that good.  (The ones that come free with audio equipment.)
> 
> -- glen
> 
> 

If you are unsure of the quality of RCA phono cables, I would suggest 
buying "video cables". These are usually good quality cables from a 
shielding point of view. Molded audio cables vary.  


-- 
Al Clark
Danville Signal Processing, Inc.
--------------------------------------------------------------------
Purveyors of Fine DSP Hardware and other Cool Stuff
Available at http://www.danvillesignal.com

bg_ie@yahoo.com (Barry) wrote in message news:<731cea69.0311180643.7a93730c@posting.google.com>...
> Hi all,
> 
> I have a Codec (2ADCs and 2DACs), which is capable of 16bit per
> channel sampling, at 96kHz. It interfaces to a DSP board. I have
> designing a pcb board capable of processing an analog music signal,
> and I'd like to interface it to an ADC on my codec. I'd also like to
> send a signal from the DAC to a second pcb board. I'm just wondering
> what would be the best way of doing this? The Codec has phono RCA
> connections. The line input on the Codec has a Signal to noise ratio
> of 90dBs VRms. (for a 1k full scall input), but this does not include
> the PCB RCA jack into which any input or output signal most pass. The
> Codec is called the TLV320AIC23EVM by TI.
> 
> I hope to encase my pcb music boards, the codec dsp processor and
> non-switching power supply in a box, as a final product - so I dont
> need long cabling between the pcbs and codec.
> 
> What's the best way of preserving my signal to noise ratio? I was
> thinking a RCA to BNC adaptor, then a short BNC cable to each pcb
> (each with a BNC input/output connector).
> 

Barry,

The SNR goes all to heck when the connection is broken.  You can
suspend a heavy box at the end of a BNC-terminated cable because the
connector is locking.  In the same situation, the RCA cable will pull
out and goodbye SNR.

There's more to design than electrons and magnetic fields!  :-)

Fred