[SBE] OK - Ohh great and wise minds

[Gary O'Guinn]

Hi William:

Tektronix has a really good White Paper on PCR jitter that explains what 
PCR is and the effect of PCR jitter. Here is a link to a paper (not the 
one from Tektronix) on PCR Jitter. 
http://www.broadcastpapers.com/whitepapers/PixelmetrixPCRJitter.pdf?CFID=36065957&CFTOKEN=ab9aed43745f836f-7F558EDC-B9A5-A400-A19D92D320293AAE

In short the *P*rogram *C*lock *R*eference signal is the "master" data 
clock for the MPEG packets. When the packets are created a PCR signal is 
included in the header. This contains a time when the packet was created 
(similar to time code in an analog frame). Since the data is transmitted 
asynchronously the receiver  has to have some method of re-clocking the 
packets to it knows where the beginning of a packet is. The PCR provides 
this clock reference to the receiver. It is similar to Horizontal and 
Vertical Sync signals in a analog video frame. The need for re-stamping 
occurs when the packets are messed with. Such as if two data streams are 
multiplexed together. Since there are time delays associated with 
multiplexing, the MUX generates a new PCR and re-stamps it into the 
packet header. This is supposed to guarantee packet synchronization for 
the decoder. Since the transmitter's modulator is the last device in the 
broadcast chain, it needs to re-clock the MPEG stream (because of the 
8VSB specification) since it has manipulated the stream it needs to re 
time stamp the PCR.

PCR jitter can be a major factor. If the jitter is too severe the clock 
reference cannot be obtained by the decoder and thus no packets will be 
decoded properly. You probably should measure PCR jitter at the 
transmitter site. We transmit the data to our transmitter site via 
microwave: as an ASI stream. At the transmitter we trans code ASI to 
SMPTE-310 for the 8VSB modulator; with a Tandberg device. We noticed 
that with the Tandberg transcoder we have out of spec PCR jitter. We 
suspect the microwave gear is causing the jitter. We had to remove the 
Tandberg and are now using a K-Tech transcoder. The K-Tech seems to be 
more forgiving and the PCR jitter is within specification using the 
K-Tech. The Tandberg device is working properly, and I don't know why it 
is less stable at the transmitter than the K-Tech.

Also check the PSIP Tables for proper timings. We use a Sencore 
Investigator and we also noticed that the PSIP tables were not being 
updated quite fast enough. So I changed the encoder settings slightly to 
increase the PSIP table update times to make sure they were being 
updated more frequently in order to maintain ATSC specs.

I hope this was what you were looking for.
Gary O'Guinn
KTUU-TV

William Whitt wrote:

>

> I have a couple of questions -- What is the point of PCR re-stamping? 

> If I was told I can't remember.

>

>  

>

> I have a digital transmitter that we use as a STL/intercity relay -- 

> Digital CH23. On the mountain top I have a Drake unit that converts CH 

> 23 to an ASI stream that I feed to my Full Power transmitter on a 

> different channel. I feed my STL/intercity relay with a steady stream 

> at 19.392Mbs from my multiplexer.

>

>  

>

> At least once a month I lose my PSIP data on some manufactures digital 

> convertor boxes. I also get weird video and audio issues ... I go up 

> to the mountain top, reset the Drake(unplug and plug back in) and poof 

> everything is back in order.

>

>  

>

> Well -- I thought there may be a problem with the STL/intercity relay, 

> maybe it was causing a drift by doing something to my ASI stream that 

> eventually screws up the Drake on the mountain. I called the 

> manufacture and they said -- the only thing that might cause that is 

> if the PCR re-stamping was turned on ... ???

>

>  

>

> I drilled into the menu of the modulator on my transmitter and sure 

> enough the PCR re-stamping was selected. Would that cause this issue? 

> It also had "Del Null Padding" which by its default was disabled. I 

> left that one alone.

>

>  

>

> Any ideas? Or is this a direct result of the digital world, and I'm 

> going to have to come up with some kind of remote reset for the Drake?

>

>  

>

> Thanks in advance,

>

>  

>

> Bill

>

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