First of all, it is ilegal for manufacturers to develop a recorder that records HD via hdmi, dvi, or component.
(Update: See end of post for correction)
If such a recorder was manufacturered, it wouldn't be very practical anyway. On those interfaces, the data rate is over 1 gb/s and even if it was possible to save the data in the native form (hard disks currently cannot substain that data rate transfer), a 2 hour program would require about 1 terabyte (1000 GB) of disk space. So a possible solution would be to include a realtime hardware encoder (minimum of £5,000) into the hardware of the recorder which would not be very cost effective. Trying to use a software encoder wouldn't be possible either since as mentioned earlier, the raw data would be required to be saved on the hard disk prior to encoding and hard disks do not operate at that speed, the disk size would have to be at least 2 terabytes, and you'd probably have to wait several hours after the content was saved before you could view the recording.
The only currently used interface for recording HD is the ilink interface (firewire) but since that has not widely accepted, a move is currently underway to use a TCP/IP interface. The ilink/firewire interface saves the compressed data from the stream (usually 20 mb/s or less data rate which is less than 1/60th the data rate and size of raw data) to disk, tape, or BD. If an STB had this interface and supported software (none currently available any place in the world) and a customer provided a ilink/firewire recorder, it may be possible to record some HD programs.
If an STB had a ilink/firewire interface (with supported software), the received stream would first be decrypted (if necessary - probably most if not all Sky HD channels) by the STB and then transmited via the ilink/firewire interface. However before it could transmit the stream via the ilink/firewire interface, the STB is required to see if copy protection is enabled (5c protection on the stream) and if not, the transmission cannot occur. Since most of the HD received by a Sky STB is probably copy protected, it is highly unlikely that recordings would be possible even if the ilink/firewire interface was available. Possibly only the BBC HD and CH4 HD may not be copy protected but since the current Sky STB doesn't support the ilink/firewire interface, it doesn't make much of a difference.
In the US and Japan it is possible to purchase ilink/firewire HD recorders (either disk or tape) and in Japan it is possible to purchase ilink/firewire Blue Ray recorders. However none of these are very practicle due to copy protection and lack of an ilink/firewire interface on any STB. These devices currently only work with a TV that is receiving HD channels via OTA (Over the Air) or possibly via cable, the TV must have an ilink/firewire interface (with software), and copy protection is disabled in the stream. To record from cable, the TV digital tuner needs to be connected via the coax connection and not through the STB. These recorders are of no use for SAT reception until manufacturers of SAT STBs start providing an ilink/firewire interface with the supported software.
In the US, most HD OTA transmissions (major national networks as well as other local channels) and cable transmissions for those same channels are usually not copy protected or encrypted so therefore most of those channels may be recorded via the TVs ilink/firewire interface. However, a content provider (usually the owner of very recent movies) may require the broadcaster to protect that content causing the broadcaster to enable 5c protection when transmitting that content (the 5c protection is enabled or disabled in the stream) not allowing the TV software for that content to be transmitted to the ilink/firewire interface.
An ilink/firewire interface on a TV will not work for data supplied by an STB since the ilink/firewire interface needs the data from the compressed stream and the STB supplies raw data to the TV via hdmi, dvi, or component.
Also it seems that the new effort to create a TCP/IP interface will not solve anything since so little content is allowed to be recorded.
So as you can see, the content providers have put heavy restrictions for recording HD content.
Enabling the external SATA or USB interface on a STB is also heavily restricted for recording HD on an external hard drive. The software design has to be approved and certified by an organization controlled by the content providers. To pass certification, it must be impossible to retrieve viewable data from the the drive when conntected to a PC or other device and the drive must not be able to be used by another STB. I know of only one manufacturer that has implemented the software to support external SATA drives and has passed certification for their DVR (Scientific Atlanta 8300 HD series) while running SARA software. If the 8300 HD DRV is running other software (eg. Passport from Panasonic), an external SATA drive will not be recognized or used.
Update: I forgot that camcorder encoder chips are fairly inexpensive so encoding via hdmi, dvi, or component would be possible though still possibly illegal. Originally I had considered only broadcaster encoder chips but those would probably not be required. However, wiring up and managing such a device that used hdmi, dvi, or component would be very cludgy and you wouldn't be able to record in the background while watching another channel since the display output is what is being recorded.
BBC HD is FTA and I suppose it's not too difficult to record that. The Sky One HD stuff is a different matter, it's encrypted and originally in 1080i (mind you, so is BBC HD although not encrypted), not 720p.
If both of those channels are unencrypted, someone can purchase a DVB-S2 PC card to receive those channels on a PC. They would also need DVR software to decode the stream to be displayed and/or save the encoded data to the disk in an encrypted form (encrypted with a random key that is only known to that computer). If the 5c copy protection is not enabled for a program, an in the clear (decrypted) copy of any saved content can also be saved to the disk.
However if the 5c protection is enabled in the stream, legal DVR software will not allow the user to copy an in the clear version to the disk. In that case, only a pirate version that has been modified to bypass the 5c copy protection would allow an in the clear version of the content to be saved.
If the received stream is encrypted, a PC cannot be used. Maybe Sky does not encrypt Sky One HD since much of its content is from the US broadcasters which are not very concerned about copy protection and therefore the stream could possibly be unencrypted and copy protection not enabled. However, that is unlikely since anyone could receive Sky One HD without a Sky subscription.
I suspect that what you thought was Sky One HD content was really content that was captured from US OTA national network broadcasters (FOX, NBC, CBS, ABC, CW, PBS, or MNTV). If you really did see the content in 720p, it was probably from FOX or ABC.
HD PVR is the worlds first High-Definition video recorder for making real-time H.264 compressed recordings at resolutions up to 1080i. HD-PVR records component video (YCrCb) from cable TV and satellite set top boxes, with a built-in IR blaster to automatically change TV channels for scheduled recordings. Audio is recorded using AAC.
The recording format is AVCHD, which can be used to burn Blu-ray DVD disks. Two hours of HD recordings, recorded at 5 Mbits/sec, can be burnt onto a standard 4.7 GByte DVD-R or DVD-RW disk for playback on a Blu-ray DVD player.