Are You Being Cheated by Digital Cable?

Lauren Weinstein writes "Even though your cable company may claim that a channel is in a digital tier that you're paying for, they may be sending it to you in analog form, with associated negative effects. Surprise! Are You Being Cheated by Digital Cable? 'You're paying for digital, you should get digital. Outside of the lower video and audio quality that can be present on many analog feeds, third-party devices (like cableCARD TiVos) which could otherwise record a digital signal directly, will be forced to re-digitize an analog signal, with inevitable quality loss in the process. But how to know for sure if a channel is digital or analog as received?'"

Comments

On Comcast it's easy

[kimvette]

On Comcast it's easy to tell analog from digital feeds: on digital cables the S/PDIF signal is present, on analog feeds it is not, so on the analog feeds I need to switch my audio receiver to use the line-level input instead of digital.

Re:On Comcast it's easy

[The Vulture]

In a typical cable company office (I've been in a couple), you'll see rows upon rows of boxes that they use to receive the actual television signals from satellite (one per channel they receive). Most of these boxes are provided by the networks in question.

Many of these boxes can only output the signal as analog (on a user-specified frequency, for arbitrary placement in the channel map), some of them are capable of outputting MPEG-2 data using ASI as the physical link. In order to cram multiple channels in one frequency, the MPEG-2 streams have to be changed (PID numbers must be changed to be non-duplicates, PAT and PMT packets need to be updated), then these MPEG-2 streams need to be muxed together and encoded into QAM.

Seeing as this is an expensive process (that cable companies might not have planned for, especially in the case of smaller operators), I believe that many of them are waiting for the migration to MPEG-4, to get the most bang for the buck.

-- Joe

Look at the noise

[Spazmania]

But how to know for sure if a channel is digital or analog as received?'

Look at the noise characteristics. Analog and digital respond to noise differently. Digital pixilates and stutters but otherwise displays a perfect picture. Analog ghosts and snows.

If you're not getting enough noise to tell the difference then smile and be happy because you have a better cable TV signal than most of the rest of us.

How to know...

[evilviper]

But how to know for sure if a channel is digital or analog as received?'"

Begin unscrewing the coax cable from your cable box. As you very, very slowly pull it away, if the signal starts to fade/shows static/etc., it's certainly analog. If, instead, it suddenly goes from perfect, to black, it's digital. Also, in the latter case, it will probably start to show artifacts, perfectly square 16x16 pixel macroblocks that stand out in sharp contrast to the rest of the picture.

Re:Analog is better here.

[InvalidError]

But that being said, there is a limited amount of bandwidth and all the channels we expect to have take up a lot of space. HD is even worse, it eats up bandwidth and processing equipment.

Bandwidth is not really a problem for HDTV: from what I read, most current HDDVD and BluRay titles are encoded at less than 10Mbps total. Since a DOCSIS modem can pull over 40Mbps from a single 6MHz NTSC channel bandwidth, a digital cable box should be able to squeeze at least three very good quality HD channels in the same bandwidth as one analog channel. With about 900MHz worth of usable downstream bandwidth on coax, there is room for up to 450 high-quality HD channels. Of course, about half of that spectrum is used by analog channels, SD/ED digital channels and cable-modems so there should still be room for 150-200 HQ-HD channels.

As for the processing equipment, the heavy-lifting is at the source where initial encoding is done and at the head-end if there is transcoding to be done. The rest is standard fare digital broadcast over an HFC network just like it is for all other digital cable broadcasts. Since head-ends already have quite a bit of equipment dedicated to each channel they support on their networks, having an extra transcoding/scaling unit in loops that require it is (usually) a minor hurdle.

Re:Very interesting ...

[jasonwea]

Non-square pixels are actually quite common. In PAL land 16:9 SD is transmitted as 576i (720x576 or 702x576). I'm guessing your SD feeds are 720x480 and need to be scaled to 16:9 (say 854x480 or similiar). This is quite normal for PAL/NTSC compatible feeds. (Again in PAL) there are other aspect ratios that are used on lower bitrate channels such as 544x576 and 480x576.

If you are using something like VLC or mplayer (or even Media Player Classic on Windows), it shouldn't be too hard to get it to look right. Most feeds should have the MPEG aspect ratio flag set and it should Just Work. Otherwise you should be able to force the aspect ratio (4:3 or 16:9) in your playback software.

Composite video has a maximum bandwidth

[tepples]

Analog stations don't have a specific "horizontal resolution" In a way, they do; it's called the Nyquist rate. Terrestrial broadcast television (M system) and standard cable television consist of amplitude-modulated NTSC composite video with a vestigial sideband, combined with frequency-modulated audio. In NTSC, 0 to roughly 3.0 MHz of a composite video signal is luma (Y), while 3.0 to 4.2 MHz is chroma (Pb and Pr, multiplexed with QAM). Nyquist's theorem states that this Y signal can hold only 6 million samples per second. There are 15734 scanlines per second, and about 82 percent of a scanline is video, the rest being horizontal blanking. This gives a total of (6000000/15734)*0.82 = 312 distinct luma samples at the Nyquist rate. Many video systems slightly oversample this to 320, which provides (desirable) square luma elements at the field rate.