HDTV storage needs

[Mark J. Dulcey]

Rich Braun wrote:
> I wrote:
>> A terabyte can hold 55 hours of raw video
> 
> I didn't do the math right, an hour of standard broadcast high-def is 7.4G so
> a terabyte holds 135 hours.  (For home videos, at least with the Sony
> camcorderI use, an hour is about 13G so a TB is 76 hours.)  Sorry 'bout the
> error.

The size of an hour of standard broadcast can vary. The maximum data 
rate of the ATSC data stream is 18Mbps, or 2.2MB per second, which comes 
to 8,100MB (about 8 gigabytes) per hour. Not all stations send a maximum 
rate data stream, and some multicast so the bandwidth to capture a 
single broadcast is lower. And at least two local DTV stations, WUNI-DT 
and WUTF-DT, only broadcast a single standard-definition signal; both of 
them use a data rate of about 8Mbps. (Those are Spanish-language 
stations and their parent networks do not yet provide HD content. Their 
Mexican supplier, Televisa, does, so we'll probably see HD on Univision 
and Telefutura someday.)

Cable and satellite systems generally use lower data rates than 
broadcast ATSC does, so you'll be able to store more hours of video if 
you record from one of those sources. On the other hand, it won't look 
as good; the HD video from the broadcasts of the major networks is very 
good indeed, at least on the programs that provide good source material. 
Dramas and single-camera comedies shot on film are excellent; 
multi-camera comedies, game shows, and reality shows shot on video are 
more variable, and some are still SD. The flagship late night shows 
(Letterman and Leno) are also very good; they were among the very first 
shows to be broadcast in HD. For now at least, some daytime shows are 
also shot in HD video and broadcast that way, but I'm not convinced that 
will last; I suspect that the long-term course of broadcast television 
will be multicasting during the day, with HD content reserved for prime 
time, sports, and the major late-night programs. Locally, Channel 5 is 
doing local news in HD; none of the other stations are.

The data rate of the most common types of digital camcorders (MiniDV, 
Digital8, and the most common variant of HDV) is 25Mbps, which is indeed 
about 13 gigabytes per hour. AVCHD cameras (most of the new flash and 
hard disk based cameras) use lower data rates (but use MPEG4 encoding 
instead of MPEG2) and most offer more than one choice, so you get a 
quality vs. recording time tradeoff.