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From: JosephKK on 29 Jul 2010 00:52
On Tue, 27 Jul 2010 00:34:12 +0100, Dirk Bruere at NeoPax
<dirk.bruere(a)gmail.com> wrote:

>On 24/07/2010 14:45, JosephKK wrote:
>> On Fri, 23 Jul 2010 16:52:36 +0100, Dirk Bruere at NeoPax
>> <dirk.bruere(a)gmail.com> wrote:
>>
>>> On 23/07/2010 05:09, JosephKK wrote:
>>>> On Tue, 20 Jul 2010 17:39:17 +0100, Dirk Bruere at NeoPax
>>>> <dirk.bruere(a)gmail.com> wrote:
>>>>
>>>>> I want to drive a digital audio signal over CAT5.
>>>>> I intend to use the DA103 transformer but need to check impedance matching.
>>>>>
>>>>> Will 100R each end of the CAT5 be suitable?
>>>>> Should the 100R on the transmitting end be before, or after, the xformer?
>>>>
>>>> The terminating restance should be at the far end of the cable. I
>>>> gotta ask though, in there any reasonable way that you could send the
>>>> signal digitally? like manchester encoded? The whole thing can work
>>>> much better that way.
>>>
>>> Forgot to add...
>>> The input signal is standard SPDIF.
>> OK.
>>> I'm going to bump it up to TTL level and use that to feed into a am26C31
>>> line driver and then into the transformer. The other end is an SPFIF
>>> receiver, so I have to do an impedance match (and voltage reduction to
>>> around 1V).
>>
>> Actually, i think all you really need is the impedance match (both
>> ends and let the voltage scale). Unless you are going a lot farther
>> than SPDIF is rated for, in which case the driver may be called for.
>
>The only worry is that the receiving chip is 3V3 and I do not want to
>risk damaging it by feeding it some overvoltage. Can bad impedance match
>bump up the voltage through reflections?

Yes it can. With this configuration and guessed line lengths it may
show up as increased overshoot / undershoot, more ringing or leading
pedistals on the pulses at the receiver.

I still recomment just Z matching both ends and just using Cat 5 / Cat
5E cable. It really is nice cable for fast signals. I know, i done
got me a copy of TIA-568 set.

>
>I checked the specs for the am26c31 and the output is around 3V (5V
>Vcc), so after the xformer and resistors it will certainly be less.
From: Dirk Bruere at NeoPax on 29 Jul 2010 07:40
On 29/07/2010 05:52, JosephKK wrote:
> On Tue, 27 Jul 2010 00:34:12 +0100, Dirk Bruere at NeoPax
> <dirk.bruere(a)gmail.com> wrote:
>
>> On 24/07/2010 14:45, JosephKK wrote:
>>> On Fri, 23 Jul 2010 16:52:36 +0100, Dirk Bruere at NeoPax
>>> <dirk.bruere(a)gmail.com> wrote:
>>>
>>>> On 23/07/2010 05:09, JosephKK wrote:
>>>>> On Tue, 20 Jul 2010 17:39:17 +0100, Dirk Bruere at NeoPax
>>>>> <dirk.bruere(a)gmail.com> wrote:
>>>>>
>>>>>> I want to drive a digital audio signal over CAT5.
>>>>>> I intend to use the DA103 transformer but need to check impedance matching.
>>>>>>
>>>>>> Will 100R each end of the CAT5 be suitable?
>>>>>> Should the 100R on the transmitting end be before, or after, the xformer?
>>>>>
>>>>> The terminating restance should be at the far end of the cable. I
>>>>> gotta ask though, in there any reasonable way that you could send the
>>>>> signal digitally? like manchester encoded? The whole thing can work
>>>>> much better that way.
>>>>
>>>> Forgot to add...
>>>> The input signal is standard SPDIF.
>>> OK.
>>>> I'm going to bump it up to TTL level and use that to feed into a am26C31
>>>> line driver and then into the transformer. The other end is an SPFIF
>>>> receiver, so I have to do an impedance match (and voltage reduction to
>>>> around 1V).
>>>
>>> Actually, i think all you really need is the impedance match (both
>>> ends and let the voltage scale). Unless you are going a lot farther
>>> than SPDIF is rated for, in which case the driver may be called for.
>>
>> The only worry is that the receiving chip is 3V3 and I do not want to
>> risk damaging it by feeding it some overvoltage. Can bad impedance match
>> bump up the voltage through reflections?
>
> Yes it can. With this configuration and guessed line lengths it may
> show up as increased overshoot / undershoot, more ringing or leading
> pedistals on the pulses at the receiver.
>
> I still recomment just Z matching both ends and just using Cat 5 / Cat
> 5E cable. It really is nice cable for fast signals. I know, i done
> got me a copy of TIA-568 set.
>
>>
>> I checked the specs for the am26c31 and the output is around 3V (5V
>> Vcc), so after the xformer and resistors it will certainly be less.

Given that the SPDIF input is 75R, a couple of 12R resistors in each
signal path at the SPDIF end should be sufficient?

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.blogtalkradio.com/onetribe - Occult Talk Show
From: Dirk Bruere at NeoPax on 29 Jul 2010 12:28
On 29/07/2010 12:40, Dirk Bruere at NeoPax wrote:
> On 29/07/2010 05:52, JosephKK wrote:
>> On Tue, 27 Jul 2010 00:34:12 +0100, Dirk Bruere at NeoPax
>> <dirk.bruere(a)gmail.com> wrote:
>>
>>> On 24/07/2010 14:45, JosephKK wrote:
>>>> On Fri, 23 Jul 2010 16:52:36 +0100, Dirk Bruere at NeoPax
>>>> <dirk.bruere(a)gmail.com> wrote:
>>>>
>>>>> On 23/07/2010 05:09, JosephKK wrote:
>>>>>> On Tue, 20 Jul 2010 17:39:17 +0100, Dirk Bruere at NeoPax
>>>>>> <dirk.bruere(a)gmail.com> wrote:
>>>>>>
>>>>>>> I want to drive a digital audio signal over CAT5.
>>>>>>> I intend to use the DA103 transformer but need to check impedance
>>>>>>> matching.
>>>>>>>
>>>>>>> Will 100R each end of the CAT5 be suitable?
>>>>>>> Should the 100R on the transmitting end be before, or after, the
>>>>>>> xformer?
>>>>>>
>>>>>> The terminating restance should be at the far end of the cable. I
>>>>>> gotta ask though, in there any reasonable way that you could send the
>>>>>> signal digitally? like manchester encoded? The whole thing can work
>>>>>> much better that way.
>>>>>
>>>>> Forgot to add...
>>>>> The input signal is standard SPDIF.
>>>> OK.
>>>>> I'm going to bump it up to TTL level and use that to feed into a
>>>>> am26C31
>>>>> line driver and then into the transformer. The other end is an SPFIF
>>>>> receiver, so I have to do an impedance match (and voltage reduction to
>>>>> around 1V).
>>>>
>>>> Actually, i think all you really need is the impedance match (both
>>>> ends and let the voltage scale). Unless you are going a lot farther
>>>> than SPDIF is rated for, in which case the driver may be called for.
>>>
>>> The only worry is that the receiving chip is 3V3 and I do not want to
>>> risk damaging it by feeding it some overvoltage. Can bad impedance match
>>> bump up the voltage through reflections?
>>
>> Yes it can. With this configuration and guessed line lengths it may
>> show up as increased overshoot / undershoot, more ringing or leading
>> pedistals on the pulses at the receiver.
>>
>> I still recomment just Z matching both ends and just using Cat 5 / Cat
>> 5E cable. It really is nice cable for fast signals. I know, i done
>> got me a copy of TIA-568 set.
>>
>>>
>>> I checked the specs for the am26c31 and the output is around 3V (5V
>>> Vcc), so after the xformer and resistors it will certainly be less.
>
> Given that the SPDIF input is 75R, a couple of 12R resistors in each
> signal path at the SPDIF end should be sufficient?

On a related issue, how important is termination over short runs with
low(ish) frequency? Audio is around 10Mhz and I doubt we will ever go
beyond 30m in length. In this case the wavelength is around the same
lenth as the cable. Of course, this is square wave and obviously there
are higher frequency components.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.blogtalkradio.com/onetribe - Occult Talk Show
From: Paul Keinanen on 29 Jul 2010 13:13
On Thu, 29 Jul 2010 17:28:06 +0100, Dirk Bruere at NeoPax
<dirk.bruere(a)gmail.com> wrote:

>On a related issue, how important is termination over short runs with
>low(ish) frequency? Audio is around 10Mhz and I doubt we will ever go
>beyond 30m in length. In this case the wavelength is around the same
>lenth as the cable.

1/4 wavelength is an impedance inverter (an open 1/4 stub acts a short
circuit, a shorted 1/4 wavelength stub acts as an open circuit).

With a typical velocity factor for CAT5 cables of 0.67, at 10 MHz, the
wavelength is about 20 m. When operating below 1/10 wavelengths, the
transmission line issues are not very critical. Thus, with 10 MHz
signals, termination should be used for cables longer than 2 m.

From: JosephKK on 29 Jul 2010 21:40
On Thu, 29 Jul 2010 12:40:24 +0100, Dirk Bruere at NeoPax
<dirk.bruere(a)gmail.com> wrote:

>On 29/07/2010 05:52, JosephKK wrote:
>> On Tue, 27 Jul 2010 00:34:12 +0100, Dirk Bruere at NeoPax
>> <dirk.bruere(a)gmail.com> wrote:
>>
>>> On 24/07/2010 14:45, JosephKK wrote:
>>>> On Fri, 23 Jul 2010 16:52:36 +0100, Dirk Bruere at NeoPax
>>>> <dirk.bruere(a)gmail.com> wrote:
>>>>
>>>>> On 23/07/2010 05:09, JosephKK wrote:
>>>>>> On Tue, 20 Jul 2010 17:39:17 +0100, Dirk Bruere at NeoPax
>>>>>> <dirk.bruere(a)gmail.com> wrote:
>>>>>>
>>>>>>> I want to drive a digital audio signal over CAT5.
>>>>>>> I intend to use the DA103 transformer but need to check impedance matching.
>>>>>>>
>>>>>>> Will 100R each end of the CAT5 be suitable?
>>>>>>> Should the 100R on the transmitting end be before, or after, the xformer?
>>>>>>
>>>>>> The terminating restance should be at the far end of the cable. I
>>>>>> gotta ask though, in there any reasonable way that you could send the
>>>>>> signal digitally? like manchester encoded? The whole thing can work
>>>>>> much better that way.
>>>>>
>>>>> Forgot to add...
>>>>> The input signal is standard SPDIF.
>>>> OK.
>>>>> I'm going to bump it up to TTL level and use that to feed into a am26C31
>>>>> line driver and then into the transformer. The other end is an SPFIF
>>>>> receiver, so I have to do an impedance match (and voltage reduction to
>>>>> around 1V).
>>>>
>>>> Actually, i think all you really need is the impedance match (both
>>>> ends and let the voltage scale). Unless you are going a lot farther
>>>> than SPDIF is rated for, in which case the driver may be called for.
>>>
>>> The only worry is that the receiving chip is 3V3 and I do not want to
>>> risk damaging it by feeding it some overvoltage. Can bad impedance match
>>> bump up the voltage through reflections?
>>
>> Yes it can. With this configuration and guessed line lengths it may
>> show up as increased overshoot / undershoot, more ringing or leading
>> pedistals on the pulses at the receiver.
>>
>> I still recomment just Z matching both ends and just using Cat 5 / Cat
>> 5E cable. It really is nice cable for fast signals. I know, i done
>> got me a copy of TIA-568 set.
>>
>>>
>>> I checked the specs for the am26c31 and the output is around 3V (5V
>>> Vcc), so after the xformer and resistors it will certainly be less.
>
>Given that the SPDIF input is 75R, a couple of 12R resistors in each
>signal path at the SPDIF end should be sufficient?

Might be. You may also try 15R or 18R to see what works best, then a
1:1 transformer right into the receiver. Resistors at both ends, it
will cost some signal amplitude, maybe 40% in voltage. (105/75)^1/2
~= 1.18 so 1.2:1 transformers are likely to work better, 1.2 side on
the Cat5 cable side.
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