On Sat, 05 Nov 2011 07:24:44 -0500, "haxorcize"
<haxorcize@n_o_s_p_a_m.gmail.com> wrote:
>>On Fri, 04 Nov 2011 15:27:50 -0500, haxorcize wrote:
>>
>>>>On 11/4/2011 5:24 AM, haxorcize wrote:
>>>>> Hi!
>>>>>
>>>>> I am trying to write a demodulator for a 6MHZ-wide QAM256 signal. I
>>>>> have captured a sample file with the aid of a USRP, and I carry on
>>> the
>>>>> rest in Matlab.
>>>>
>>>>Be clear about this. Is the interesting part of the signal 6 MHz, or
>the
>>>>whole signal? Signals that are outside the band of interest will create
>>>>aliases if the sampler can see them.
>>>>
>>>>Jerry
>>>>--
>>>>Engineering is the art of making what you want from things you can get.
>>>>
>>>>
>>> The interesting part.
>>> That's the bandwidth of a single downstream MPEG channel. Every 6MHz
>>> there's a new channel.
>>
>>Are you getting a channel that has all the other channels filtered out,
>>or are you getting a channel that comes complete with scrud at the edges?
>>
>>You can't just sample based on your signal's content - you have to sample
>
>>such that your signal after aliasing and filtering is clear.
>>
>>This may help:
>>http://www.wescottdesign.com/articles/Sampling/sampling.pdf
>>
>>--
>>www.wescottdesign.com
>>
>
>I sample at 8Msps (USRP's decimation factor 8), which causes the USRP to
>give me an 8MHz-wide band. From that I use a regular FIR filter in Matlab
>to reduce it to 6MHz. I can see it 8MHz wide before the filter, and 6MHz
>afterwards. No other frequencies inside. There's an antialiasing filter
>inside the USRP so I can be sure I only get an 8MHz-wide "strip" to begin
>with.
That depends on the original sample rate, the signals across the total
input bandwidth, and how the decimation was done.
Eric Jacobsen
Anchor Hill Communications
www.anchorhill.com
Reply by haxorcize●November 5, 20112011-11-05
>On Fri, 04 Nov 2011 15:27:50 -0500, haxorcize wrote:
>
>>>On 11/4/2011 5:24 AM, haxorcize wrote:
>>>> Hi!
>>>>
>>>> I am trying to write a demodulator for a 6MHZ-wide QAM256 signal. I
>>>> have captured a sample file with the aid of a USRP, and I carry on
>> the
>>>> rest in Matlab.
>>>
>>>Be clear about this. Is the interesting part of the signal 6 MHz, or
the
>>>whole signal? Signals that are outside the band of interest will create
>>>aliases if the sampler can see them.
>>>
>>>Jerry
>>>--
>>>Engineering is the art of making what you want from things you can get.
>>>
>>>
>> The interesting part.
>> That's the bandwidth of a single downstream MPEG channel. Every 6MHz
>> there's a new channel.
>
>Are you getting a channel that has all the other channels filtered out,
>or are you getting a channel that comes complete with scrud at the edges?
>
>You can't just sample based on your signal's content - you have to sample
I sample at 8Msps (USRP's decimation factor 8), which causes the USRP to
give me an 8MHz-wide band. From that I use a regular FIR filter in Matlab
to reduce it to 6MHz. I can see it 8MHz wide before the filter, and 6MHz
afterwards. No other frequencies inside. There's an antialiasing filter
inside the USRP so I can be sure I only get an 8MHz-wide "strip" to begin
with.
Reply by Mark●November 4, 20112011-11-04
>
> >I am trying to demodulate DVB-C (DOCSIS/HDTV over cables).
> >The packets in this stream are 188 bytes long.
> >What you said about syncing on this single byte is indeed hard since it's a
> >single symbol - but I just thought that this byte repeats itself every 188
> >symbols.
> >What if I adjust my symbol timing offset based on this every-188-symbols
> >repetition, and when I feel like it's stable I got the offset for all the
> >symbols.
>
> >I have to admit it doesn't sound solid. I am looking for literature on
> >cable modems to see how they work. Thanks.
>
>
if you are doing this as a learning experience, ok go for it...
if you really want to get this to work, i don't want to be
discouraging but you have a long way to go... 256QAM over cable has
little chance of working without adaptive equalization..
my suggestion is to pick apart one of the many commercially avaialbe
chips that do this...
or start with QPSK and work up through 16QAM to 64QAM then 256QAM.
Mark
Reply by Eric Jacobsen●November 4, 20112011-11-04
On Fri, 04 Nov 2011 15:35:52 -0500, "haxorcize"
<haxorcize@n_o_s_p_a_m.gmail.com> wrote:
>>On Fri, 04 Nov 2011 04:24:33 -0500, haxorcize wrote:
>>
>>> Hi!
>>>
>>> I am trying to write a demodulator for a 6MHZ-wide QAM256 signal. I
>have
>>> captured a sample file with the aid of a USRP, and I carry on the rest
>>> in Matlab.
>>>
>>> [Question regarding sampling frequency]: I get max 8MS/s from the USRP
>>> (although the spec says it should support 16MS/s - I can't seem to get
>>> that for some reason). It seems like I should take >12MS/s to avoid
>>> aliasing for a 6MHz signal, but I've read somewhere that if I take I&Q
>>> samples, it can count like two samples. Is it true? Will 8MS/s be
>>> sufficient to properly work with a 6MHz wide signal?
>>
>>You need to collect _independent_ samples at over twice the signal
>>bandwidth; they don't have to be evenly spaced, or the same thing, etc.
>>Since I and Q channels are independent (and useful), that counts. I
>>don't know of any systems that collect anything other than just straight,
>
>>or I and Q, but I'm sure that someone has tried.
>>
>>> [Question regarding demodulator design]: The channel contains MPEG2-TS
>>> frames, and they contain an sync byte (=0x47).
>>> I thought I could use this for symbol timing recovery with correlation.
>>> (a) Do you know it's good enough?
>>> (b) Is it possible to do symbol timing recovery with correlation before
>>> I corrected carrier freq/phase offsets? (c) Suppose I do have to fix
>>> carrier freq/phase offsets - will a costas loop be okay?
>>
>>I've never tried QAM256, so take this with a grain of salt:
>>
>>I don't think you can use your sync byte for symbol timing recovery -- it
>
>>is, after all, just one point in the constellation; if you haven't locked
>
>>on the carrier and the bit timing, then how can you capture that byte?
>>
>>Study your signal specification. I'd be surprised if they're not helping
>
>>you out somehow with some sort of a periodic training or sync sequence
>>(no 8-symbol bursts of 0xff, 0x00, 0xff?, for example?). Whether or not
>>they do, some sort of data-directed synchronization (e.g. Costas) is
>>probably the way to go.
>>
>>Have you done a literature search? What did it find? If this is part of
>
>>a specific service, then look for books on the subject -- sometimes
>>that's your best bet for pointers of this sort.
>>
>>--
>>www.wescottdesign.com
>>
>
>I am trying to demodulate DVB-C (DOCSIS/HDTV over cables).
>The packets in this stream are 188 bytes long.
>What you said about syncing on this single byte is indeed hard since it's a
>single symbol - but I just thought that this byte repeats itself every 188
>symbols.
>What if I adjust my symbol timing offset based on this every-188-symbols
>repetition, and when I feel like it's stable I got the offset for all the
>symbols.
>
>I have to admit it doesn't sound solid. I am looking for literature on
>cable modems to see how they work. Thanks.
You're neglecting the overhead due to FEC encoding. The MPEG framing
often winds up not being synchronous with the symbols as it generally
doesn't need to be. I don't recall whether it does with DOCSIS or
not, but either way since the signal is continuous and scrambled you
don't really need the framing markers for synchronization.
Eric Jacobsen
Anchor Hill Communications
www.anchorhill.com
Reply by Tim Wescott●November 4, 20112011-11-04
On Fri, 04 Nov 2011 15:27:50 -0500, haxorcize wrote:
>>On 11/4/2011 5:24 AM, haxorcize wrote:
>>> Hi!
>>>
>>> I am trying to write a demodulator for a 6MHZ-wide QAM256 signal. I
>>> have captured a sample file with the aid of a USRP, and I carry on
> the
>>> rest in Matlab.
>>
>>Be clear about this. Is the interesting part of the signal 6 MHz, or the
>>whole signal? Signals that are outside the band of interest will create
>>aliases if the sampler can see them.
>>
>>Jerry
>>--
>>Engineering is the art of making what you want from things you can get.
>>
>>
> The interesting part.
> That's the bandwidth of a single downstream MPEG channel. Every 6MHz
> there's a new channel.
Are you getting a channel that has all the other channels filtered out,
or are you getting a channel that comes complete with scrud at the edges?
You can't just sample based on your signal's content - you have to sample
such that your signal after aliasing and filtering is clear.
This may help:
http://www.wescottdesign.com/articles/Sampling/sampling.pdf
--
www.wescottdesign.com
Reply by haxorcize●November 4, 20112011-11-04
>On Fri, 04 Nov 2011 04:24:33 -0500, haxorcize wrote:
>
>> Hi!
>>
>> I am trying to write a demodulator for a 6MHZ-wide QAM256 signal. I
have
>> captured a sample file with the aid of a USRP, and I carry on the rest
>> in Matlab.
>>
>> [Question regarding sampling frequency]: I get max 8MS/s from the USRP
>> (although the spec says it should support 16MS/s - I can't seem to get
>> that for some reason). It seems like I should take >12MS/s to avoid
>> aliasing for a 6MHz signal, but I've read somewhere that if I take I&Q
>> samples, it can count like two samples. Is it true? Will 8MS/s be
>> sufficient to properly work with a 6MHz wide signal?
>
>You need to collect _independent_ samples at over twice the signal
>bandwidth; they don't have to be evenly spaced, or the same thing, etc.
>Since I and Q channels are independent (and useful), that counts. I
>don't know of any systems that collect anything other than just straight,
>or I and Q, but I'm sure that someone has tried.
>
>> [Question regarding demodulator design]: The channel contains MPEG2-TS
>> frames, and they contain an sync byte (=0x47).
>> I thought I could use this for symbol timing recovery with correlation.
>> (a) Do you know it's good enough?
>> (b) Is it possible to do symbol timing recovery with correlation before
>> I corrected carrier freq/phase offsets? (c) Suppose I do have to fix
>> carrier freq/phase offsets - will a costas loop be okay?
>
>I've never tried QAM256, so take this with a grain of salt:
>
>I don't think you can use your sync byte for symbol timing recovery -- it
>is, after all, just one point in the constellation; if you haven't locked
>on the carrier and the bit timing, then how can you capture that byte?
>
>Study your signal specification. I'd be surprised if they're not helping
>you out somehow with some sort of a periodic training or sync sequence
>(no 8-symbol bursts of 0xff, 0x00, 0xff?, for example?). Whether or not
>they do, some sort of data-directed synchronization (e.g. Costas) is
>probably the way to go.
>
>Have you done a literature search? What did it find? If this is part of
>a specific service, then look for books on the subject -- sometimes
>that's your best bet for pointers of this sort.
>
>--
>www.wescottdesign.com
>
I am trying to demodulate DVB-C (DOCSIS/HDTV over cables).
The packets in this stream are 188 bytes long.
What you said about syncing on this single byte is indeed hard since it's a
single symbol - but I just thought that this byte repeats itself every 188
symbols.
What if I adjust my symbol timing offset based on this every-188-symbols
repetition, and when I feel like it's stable I got the offset for all the
symbols.
I have to admit it doesn't sound solid. I am looking for literature on
cable modems to see how they work. Thanks.
Reply by haxorcize●November 4, 20112011-11-04
>On 11/4/2011 5:24 AM, haxorcize wrote:
>> Hi!
>>
>> I am trying to write a demodulator for a 6MHZ-wide QAM256 signal.
>> I have captured a sample file with the aid of a USRP, and I carry on
the
>> rest in Matlab.
>
>Be clear about this. Is the interesting part of the signal 6 MHz, or the
>whole signal? Signals that are outside the band of interest will create
>aliases if the sampler can see them.
>
>Jerry
>--
>Engineering is the art of making what you want from things you can get.
>
The interesting part.
That's the bandwidth of a single downstream MPEG channel.
Every 6MHz there's a new channel.
Reply by Jerry Avins●November 4, 20112011-11-04
On 11/4/2011 5:24 AM, haxorcize wrote:
> Hi!
>
> I am trying to write a demodulator for a 6MHZ-wide QAM256 signal.
> I have captured a sample file with the aid of a USRP, and I carry on the
> rest in Matlab.
Be clear about this. Is the interesting part of the signal 6 MHz, or the
whole signal? Signals that are outside the band of interest will create
aliases if the sampler can see them.
Jerry
--
Engineering is the art of making what you want from things you can get.
Reply by haxorcize●November 4, 20112011-11-04
>"haxorcize" <haxorcize@n_o_s_p_a_m.gmail.com> wrote in message
>news:ZJadnWnx_LjMMi7TnZ2dnUVZ_uydnZ2d@giganews.com...
>> Hi!
>>
>> I am trying to write a demodulator for a 6MHZ-wide QAM256 signal.
>> I have captured a sample file with the aid of a USRP, and I carry on
the
>> rest in Matlab.
>>
>> [Question regarding sampling frequency]:
>> I get max 8MS/s from the USRP (although the spec says it should support
>> 16MS/s - I can't seem to get that for some reason).
>> It seems like I should take >12MS/s to avoid aliasing for a 6MHz
signal,
>> but I've read somewhere that if I take I&Q samples, it can count like
two
>> samples. Is it true? Will 8MS/s be sufficient to properly work with a
6MHz
>> wide signal?
>>
>> [Question regarding demodulator design]:
>> The channel contains MPEG2-TS frames, and they contain an sync byte
>> (=0x47).
>> I thought I could use this for symbol timing recovery with correlation.
>> (a) Do you know it's good enough?
>> (b) Is it possible to do symbol timing recovery with correlation before
I
>> corrected carrier freq/phase offsets?
>> (c) Suppose I do have to fix carrier freq/phase offsets - will a costas
>> loop be okay?
>
>
>Small question: are You doing DVB-T2 demodulator? There are way much more