TV Technology - Part 4 - Digital TV PIDs And Bandwidth

hello and welcome to the next video in

my series on television technology in

this video I'm going to have an in-depth

look at what's contained in the digital

bit stream that gets transmitted from

the TV station now in order to do that

I'm going to be doing everything on a

Linux host I'm going to control it from

the windows box here but everything's

done on the Linux host in another room

now in the previous video I showed how

to install the DVB adapter in Linux and

I'm going to be drawing on that for most

of this video so if you want to learn

how to get set up in order to do this

just watch the previous video

now all the examples I'm going to do I'm

just kind of based on ABC so let's rip

into it now now in the previous video we

made a channels comm file which

contained all the TV stations that are

transmitted on all the frequencies now

as i said i'm only going to concentrate

on ABC for this video so i just want to

focus on these first few here so what

i'll do is i'll just narrow this down a

bit

grep just for things in that frequency

channels abc.com okay so now it's just a

bit smaller there okay so what does it

mean first of all we have the TV station

name obviously the frequency it came in

on in version the bandwidth in this case

seven Meg the forward error correction

for these two the constellation which in

this case is a 64 qualm qualm 64

transmission mode is 8k the guard

interval is 1/16 there's no hierarchy

and these three values here are the pidz

that we're going to look closely at this

one here at the end represents the pit

for this ABC News okay these two the

first one is the video pit and the

second one is the audio feed okay so if

we go along we see ABC has 577 ABC

comedy has five seven eight ABC me 580

and we go to eight we've got another ABC

so we've got two just

ABC ones here even though they've got

different pins for their actual program

designator the video pins are the same

the audio peds are the same so it's the

same channel it's just there twice and

then we've got ABC HD and the last two

are radio

doubleday and ABC jets that's why their

video peed is zero okay because it's

just audio obviously

now when I say ped that's a packet

identifier within an MPEG transport

stream so what I'll do now is I'll tune

in ABC's frequency and we'll have a

closer look at the pigs that are

contained within it okay so the tune in

one of the channels using the adapter we

make a directory called T's F and we

copy the channels config file that we

created to that directory now we just

run the T's app command with one of

these station names just it doesn't

matter which one it just Tunes the

frequency so I'll just do T's app ABC

and what that does is tune the adapter

in now I'll just go through this briefly

signal self-explanatory that's the

signal coming in now those numbers are

kind of arbitrary it's just a bigger one

is better but they can be different from

adapter to adapter your signal to noise

which you'd hope is pretty stable which

this is a bit error rate which ideally

is zero but low is also good because we

can overcome some errors uncorrect a

blocks that's also zero or pretty low

and this is the important one as well

front end has locks so that's the front

end of the receiver so it's locked

we've got good singles noise ratio very

low errors so the signal looks pretty

good

now that program T's app simply Tunes

the adapter in so we can use it and I'm

going to install another program called

DVD snoop which lets us have a closer

look at what's going on so I'll do that

now and it's only a small program DVD

snoop ok it's in there now so just

maximize this we don't really need to

see what's at the bottom it's still

running though so now that that's

installed we can go DVB snoop and do a

scan and do a pit scan now this takes a

little while to run but you can save it

to an output and just reference it from

future if you want so I'll let it run

first to say you can see what's involved

and here it comes

so all these peers these packet

identifies are spoke about yes this DVB

snoop tells you what they're all about

so I'll just let that finish okay that's

done now and I've got a list of all the

pins that are in that transport stream

and as I said I've just also put the

output in a file just so I can reference

it easy without having to wait for a new

scan every time I want to refer to it

now we've done a pig scan we can have a

closer look at what's inside it so I'll

start with pin zero which you can see is

this program association table now we'll

just do a DVB snoop on that and there we

get lots of stuff now that's constantly

coming in right so I'll just stop that

now if you have a look at these numbers

here 5 7 6 5 7 7 5 7 8 they are the same

as up here five seven six five seven

five seven eight from the channels comp

file that we made now if we look at the

first one just use that an example 5 7 6

is ABC News so if we go back here and

see 5 7 6 for that program we see

there's a program map with this pig

which in hex is that so we go DVB snoop

on that one for program number 5 7 6 and

we see all this stuff and again that's

this constantly coming in so I'll just

stop that and scroll up to the start of

it and have a bit of a look around so

what do we see well we can see some pigs

within that and we'll start with one

that says video ok so that that says

that contains video there and next one

says that's audio so it's mapping these

different video and audio streams

together to make the program stream for

ABC News the next one what's this one

it's telly text ok that also comes in so

all of those are grouped together with

that overall pin that is the TV channel

now another thing you can do with DVD

snoop is look at the bandwidth of

individual pins now what I'm going to do

is look at the video ped for ABC News

which is this one here man this is in

decimal in the channels Khan file so the

hex version of that is 9 0 a so just DVD

snoop S which is the scan type bandwidth

and the pit so if I run that there you

can see the bandwidth live of what's

coming in for that video paid for ABC

News Now that's good and well but there

is another program called DVB traffic

which is much better at this I think so

if I run that it presents it nicely so

I'll just pause that and you can see

nine zero a which is here you can see

the same thing you see the bandwidth

here but you can see all of them in real

time and the interesting one is down the

bottom mm which is actually a dummy pin

but it represents the whole bit stream

and it shows me that we've basically got

23 megabits a second of digital data

coming in now if you remember back when

I showed the RF side of this the 7

megahertz of RF bandwidth and this shows

that in that seven megahertz of RF

bandwidth we can get 23 megabits per

second of digital bandwidth now what

I've just shown you there is MPEG

transport stream and what you've

probably mostly dealt with on computers

is MPEG program stream and the big

difference is with a normal impact file

or something you can just ask for the

file and you get it from a network or a

drive or something but with transport

stream is designed for things like TV

where it's just transmitted and you

can't ask for you know if you've missed

a few packets you can't get a

retransmission because it's over its

you're just getting sent stuff all the

time

so we've MPEG transport stream you can

have well they do have extra bits in

there for error correction which is what

you might have saw when I showed that

channels comm file some of the error

correction parameters

that's just so it can recover if there's

a few errors now obviously if there's

too many errors you'll get errors on the

screen but it can recover from a few

errors due to those extra bits that get

transmitted now another thing they do

with the TV stations is instead of just

saying okay this channel can have this

much bandwidth that one can have that

much bandwidth they know that they've

got 23 mega overall or whatever that God

so they use a statistical multiplexer to

encode the right bitrate for the type of

video that's on each programme so for

instance if you have a sports program

there's usually lots of movements so the

picture on the screen is constantly

changing so it's generally higher

bandwidth required for that as opposed

to say a newsreader who's just sitting

there like let's say I am now where

there's not much of the scene changing

so it would be a lower bit rate

so what a statistical multiplexer will

do it'll take it frame by frame and say

okay we need extra bits for this because

that's got a moving scene and that's not

so much and then and then it sends the

whole transport stream out of the

transmitter and that's what ends up here

we do multiplex it get the program

streams and we watch TV so that's what

the TV stations put in there digital bit

stream that they transmit now in the

next video I'll show you how we can get

that and use it to create an IP

multicast group to create IPTV over a

computer network so check that out