The WeaKnees Blog has posted an overview of the different DVR options to use with DirecTV HD, the HR10-250 (TiVo), HR20 (NDS) and HR21 (NDS) DVRs. It is a pretty good write-up, but there are a couple of mistakes, or places where they’ve conflated issues.
And this unit could also receive OTA HD channels – which are uncompressed, and therefore the highest quality HD broadcasts you can get, if you can get them.
This is not true. OTA HDTV is compressed using MPEG-2. (There are further encoding tricks used to squeeze the HD signal into the available 6MHz band.) And if the channel decides to use their band for multiple channels – remember digital OTA channels can have up to six sub-channels – they may turn up the compression level to squeeze the HD content into less of the bandwidth, to allow for the sub-channels to be carried. So OTA HDTV is compressed, and it is compressed to varying degrees.
Now, the claim that OTA HDTV are the highest quality HD broadcasts you can get, well, that’s debatable. Generally it is probably true. Most of the OTA HD channels, at least during prime time, devote the full available bandwidth to one HD signal and compression isn’t very high. But many cable providers carry the OTA digital signal as-is on their networks. So you may get exactly the same HD signal over cable as OTA. It is easier for the cable provider to just pull in that MPEG-2 stream and stick it on the cable as-is than it is to decode it and then re-encode it. And since broadcast channels are only a handful of their lineup, it is no big deal. So OTA often has no quality advantage over cable, with the very same signal carried by both.
Satellite, on the other hand, has more bandwidth restrictions. DBS providers have been known to use resolutions such as 1440×1080 instead of the full 1920×1080, and to use higher compression levels. So in a side-by-side comparison, the satellite signal is probably not going to look as good as OTA. But is the difference so great as to really be noticed in normal use? That probably depends on the size of the TV and the viewer, but for most users it probably isn’t such a big deal.
They also decided to eventually add new HD channels in a more compressed format called MPEG4 to allow for more channels to be sent simultaneously. Obviously, more compression means a lower quality signal, so the MPEG4 channels are often called “HD Lite” – better than SD, but not quite as good as MPEG2, and even further away from OTA quality.
This just conflates several issues and is flat out incorrect for the most part. Does MPEG-4 provide more compression than MPEG-2? Yes. Does that mean a lower quality signal? Absolutely NOT. MPEG-4 is simply a newer, more efficient compression algorithm. For the same image quality, MPEG-4 will produce a lower bit rate, requiring less bandwidth aka higher compression. And at the same bit rate, MPEG-4 provides a higher quality image. The reason satellite providers, and now cable providers, are switching to MPEG-4 from MPEG-2 is because they can get the same image quality with less bandwidth.
Now, can MPEG-4 channels look worse than an MPEG-2 channel? Of course, but it has nothing to do with the compression algorithm and everything to do with the level of compression. Turn up the compression on MPEG-2 and the image quality drops. Same with MPEG-4. A general rule of thumb is MPEG-4 is twice as good as MPEG-2 – resulting files are half the size for the same quality. But you can always turn up the compression, and say produce an MPEG-4 file 1/4 the size of the MPEG-2 file. Odds are it isn’t going to look as good as the MPEG-2 file, of course. But that’s not MPEG-4 vs. MPEG-2 – that’s higher relative compression vs. lower relative compression.
More people are more familiar with audio compression than video compression, due to iPods, Zunes, etc. Consider MP3 vs AAC. MP3 is short for MPEG-1 Layer 3 audio, and it is a fairly early audio codec. AAC is short for MPEG-4 Advanced Audio Coding, a much newer and more efficient codec. The AAC files sold through iTunes are mostly 128kbps AAC and most users think they sound pretty good. But most users think a 128kbps MP3 is low quality, and to be as good as a 128kbps AAC track the MP3 needs to be 192kbps or even 256kbps. Many people have trouble distinguishing 256kbps AAC from uncompressed audio, while MP3 needs 384kbps-512kbps, or even higher, for the same performance. The MP3 codec is less efficient, and therefore requires more bits to produce the same audio quality as the more efficient AAC encoding. It is just a different way to encode the source signal, which requires fewer bits.
Also consider that MPEG-4, specifically MPEG-4 AVC (Advanced Video Coding) aka H.264, is one of the codecs used on the best sources of HD video available today – Blu-ray Disc and HD DVD. Both formats support three codecs – MPEG-2, MPEG-4 AVC, and VC-1 (a standardized version of WMV9, which is of similar performance to MPEG-4 AVC). Most of the titles available on both formats use AVC or VC-1, and they’re considered to be the best HD content available. The quality has more to do with the encoding than the specific codec used. Since the discs have finite storage space, and further limits on transfer rates, etc, in the specs, studios use AVC because it allows them to store more content at a higher quality than MPEG-2, generally speaking.
Saying that because MPEG-4 allows more compression ‘obviously’ means a lower quality signal is just plain wrong. MPEG-4 allows more compression (than MPEG-2) for the same quality signal. Now, the actual quality will vary channel to channel simply due to different levels of compression. DirecTV, and DISH Network, are known to compress less popular channels more than the popular ones, reserving transponder bandwidth for the popular content. And it is entirely possible to have MPEG-4 channels that look better than MPEG-2 channels, when you have MPEG-4 will lower compression levels and MPEG-2 with higher compression levels. For the same bit rate, MPEG-4 will look better. There is nothing ‘HD Lite’ about the new MPEG-4 channels.
I was disappointed to see these statements, since WeaKnees is normally more accurate than that. But the write-up, overall, is OK.
