What's 'SD', 'letterbox' and 'HD/AVCHD' ?
SD ('Standard Definition') and 'letterbox' DVD's contain the same 'resolution' video data, however letterbox pixels are more 'squashed up' than SD. The data is always 620x576 (for PAL) and the only difference is the display aspect ratio 'flag', which is set to 4:3 for 'square' playback or 16:9 for 'letterbox' playback. The Video is encoded in MPEG-2 with Audio in AC-3 (5.1) format.
HD / AVCHD ('High Definition') is 1920×1080 (16:9), always square pixel. The video is encoded in h.264 (aka AVC/MPEG-4) with Audio in AC-3 (or sometimes AAC) 5.1 (or better). Note that the term 'AVCHD' is used to mean a 'Blu-ray player compatible' movie recorded on a DVD, whilst HD is used 'generically' to refer to any h.264 (or sometimes VC-1) .mp4 video file
Because h.264 encoding is more efficient than mpeg-2, HD movies are typically no more than 4 times the size of SD (rather than 5 times, as the pixel ratio would suggest). True HD movies are essentially restricted to Blu-ray disks (much as SD is restricted to standard DVD's) although MOST Blu-ray (or 'BD') players will play AVCHD from a DVD (some will even play 'raw' .mp4 files from a 'computer format' DVD !)
The big difference between DVD (SD) and BD (HD) discs is in the approach taken with DRM 'encryption'
Why is DRM different for HD ?
To "prevent" copying, of course :-)
Standard DVD movies were encrypted using a '40 bit' key system known as 'Content Scrambling System' (CSS). Unfortunately (for the proponents of DRM) by adopting a 'secret' proprietary algorithms, the structural flaws in CSS were not discovered until after it was 'released' to (i.e. imposed on) the users. These flaws reduced the effective key length to about 16 bits, allowing CSS DVD's to be 'compromised' using a simple 'brute force' attack in less than 1 minute on a 450 MHz processor (or in a few seconds, i.e. 'on the fly', by todays multi-core multi-GHz CPU's). As a result, CSS decrypting software (such as 'DVD Decrypter' and 'AnyDVD') soon became common, as users chose to bypass the 'region-specific' limitations and 'user operation prohibitions' (so they could now 'fast forward' past the accusations of piracy and annoying time-wasting adverts found before the start of the typical movie). These days, many PC owners simply install the 'DVD43' driver and then forget all about the DRM on their movie DVD's
Needless to say, the movie industry was not happy that it's 'customers' could bypass the Region Code on DVD's and purchase their movie DVD's from the cheapest world-wide source. So rather than focus on reducing costs or developing new distribution channels via the Internet (or even removing the restrictions and enforced viewing that was upsetting their paying customers), the industry decided to squander billions developing and implementing an 'uncrackable' DRM system for the 'next generation' HD movies on Blu-Ray discs
So, for HD movies, a more 'robust' encryption system was developed. Further, since HD movies are sent to the display in digital form, 'anyone' (i.e. the customer) who gained access to this data-stream could, at least 'in theory', copy the movie, so the industry came up with an approach designed to prevent player decoded movie content being 'snooped' and cloned 'on the fly'. To achieve this, each component between the Blu-ray disc (player etc) and the final display had to 'negotiate' with the next component in the 'chain' to ensure it was 'authorised' to receive the data before passing it on. Once again, all this 'DRM' was solely targeted at the paying customer (DRM can never prevent the 'fakers' making exact "bit by bit copies" of an original BD disc)
Has the HD DRM been 'cracked' ?
Of course. The HDMI digital interconnection 'secure hand-shake' system was the first to be 'broken' (it could never be kept 'secret' when hundreds of different systems had to 'talk' to each other successfully). In fact, the complexity of the 'negotiation' (and thus the diversity of 'keys' required) was most likely the main reason for it's speedy compromise. For details, see HDCP.
Further, as with all DRM, encryption is essentially pointless, since, obviously, the movie data has to be 'decrypted' into a viewable form somewhere in the 'chain'. Worse, the more complex the algorithm, the harder it is to implement in a 'closed' custom silicon chip form - and the HD DRM algorithm's were exceedingly complex !
Since many DVD player features (such as USB support, network connectivity, DLNA etc) were already being implemented in software running on Linux-based computers, 'all' the manufacturers had to do for BD players was to implement HD DRM in software - with the result that their HD 'decrypt keys' were also 'hiding' in their software. Further, as HD became 'the norm', customers were no longer 'tied' to purchasing their movies in physical BD (blu-ray) disc form - both FreeView and FreeSat offer HD, as do most of the Internet based 'catch-up TV' services - this meant that HD decrypt keys had to be present in the 'receiving' computer software (or the HD content had to be transmitted in non-encrypted form) Needless to say, with so many systems containing 'keys' in software it was inevitable that a 'key' would indeed soon be discovered. At that point, it became possible to decrypt all existing HD movies directly using software on a computer.
The industries usual heavy handed attempts in trying to prevent the 'spread' of the key only guaranteed it became 'public knowledge' in double quick time (see the AACS encryption key controversy).
One 'clever' trick, intended to improve the 'longevity' of the DRM, was that the system had multiple decrypt keys. The idea was that if one key was 'discovered' it could be set as 'invalid' on (future) physical media
However, once the system was 'compromised', the industry realised that 'invalidating' the 'lost' key would mean facing legal action from customers who could no longer play future movies on their legitimately purchased players.
Worse, from the industry point of view, was a major flaw in the system that meant once one key became known it proved to be a simple matter to 'uncover' multiple others
What are the restrictions imposed by DRM ?
Whilst the intent was to 'prevent piracy', as usual, the industry proved unable to avoid upsetting it's legitimate customers by imposing other restrictions at the same time.
1) The first of these was a repeat of the DVD 'regional coding' approach. This is intended to allow the same material to be sold at different prices in different markets (by preventing media purchased in the 'cheaper' market from being played on players in the 'expensive' market').
Whilst customers might have understood that the costs of translation for the non-USA market had to be taken into account, in fact the industry typically sold at lower prices in the '3rd world' whilst charging UK customers a price 'premium' over the US levels (for the same i.e. English language version). Needless to say, this did not make the movie industry very 'popular' with UK customers :-)
2) Another major annoyance for those with older home cinema 'surround sound' systems was the refusal of their new Blu-ray player to send 5.1 digital surround sound out via SPDIF. HD DRM only permits full AAC 5.1 decode to be sent via the HDMI cable, although most players will 'degrade' the 5.1 to 'stereo' for output to a 'headphone socket' (and this is likely what you will get if your blu-ray player has a SPDIF socket)
Some people were lucky enough (or rich enough) to purchase a HD TV with an integrated 5.1 sound system - others had to replace their old home cinema system with a new one supporting HDMI - and yet others started looking for ways to defeat the DRM.
Those wishing to retain their old SPDIF driven 'surround sound' (eg because it was part of a built-in Home Cinema system) had no choice but to 'pirate' their HD movies and reprocess the audio before playing it
3) The final 'nail in the coffin' of the movie industry for many customers was the enforced viewing of 'anti-piracy' messages at the start of the movie .. something that ONLY the legitimate customer would actually be forced to view (the 'pirates', of course' simply cut this garbage out).
If that wasn't bad enough, many studios released Blu-ray movies that enforced the viewing of 'coming soon' and other advertisements before allowing the user to play the movie. This incensed many customers who vehemently objected to being 'forced fed' spam by a movie they had actually paid for. When they discovered friends with a 'pirate' download were not being subjected to such restrictions, the writing was on the wall ..
Thus, I would suggest, the main effect of enforced viewing of 'anti-piracy' messages and spam advertising was to encourage paying customers to search for 'pirate copies' and resulted in more, rather than fewer, 'Torrent' downloads
How are broadcast movies recorded by DVR's ?
Whilst 'Sky' transmissions are all encrypted (and stored in encrypted form) this is to enforce continued subscription payments and not specifically aimed at the prevention of copying. HD movies transmitted on FreeView / FreeSat however, are typically transmitted un-encrypted, with encryption being 'reserved' for the 'pay to view' content. As a result, it became immediately obvious that the typical DVR was not limited to handling encrypted material.
In fact, DVR (digital video recorder) makers wanting a licence to support HD recording were 'required' to encrypt the (unencrypted) HD TV transmissions prior to storing them on disk (the idea being to prevent the user from 'copying' the recorded program by simply moving the hard disk or copying the file to a computer). Since the makers of DVR's used cheap 'SOC' (System On a Chip) controllers combining a low-power CPU with a simple 'GPU' (to do the actual H.264 / AC-3 decoding) any 'real time' HD encryption/decryption 'mechanism' could never be very 'complex'. Since almost all DVR's also support some means of firmware updates users soon found ways of loading 'custom' software to remove the 'encryption'
Those with older 'home cinema' systems routinely decrypt their HD recordings, upload the file to their PCs where they convert from h.264+AAC(5.1) into h.264+AC-3(5.1). After downloading the file it is then possible to use the DVR to play back the recording with full 5.1 'surround sound' sent to the SPDIF socket. Some users strip out the unwanted 'advertising' spam etc. as the same time ...
How does DLNA 'help the hacker' ?
Most modern DVR's can act as a DLNA 'server' (i.e. as a 'source' of movies etc). Since movies sent to a DLNA 'display' (typically called a 'renderer' or 'client') are sent as a simple display data stream (and it's quite possible to use a 'modern' web browser running on a PC as a DLNA 'display'), all you need to do is 'point' the DVR at your PC (the 'secure handshake' system, designed to prevent this, having been 'broken' years ago). From there it's 'just' a matter of re-encoding the incoming data stream as h.264 and saving it as a file (using, for example, VLC)
There was a time when the data rates and file sizes involved would have defeated simple display data stream copying - however today's PC's and hard disk capacities are more than up to the job.
Will HD ('AVCHD') recorded to a DVD play back on a BluRay player ?
Yes, almost all Blu-ray players are compatible with AVCHD written on DVD-R media, as well as AVCHD on BD-R media. Some 'late model' DVD players might also support AVCHD DVD's however most 'DVD only' players are mpeg-2 only (and are unable to play back h.264 HD material)
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+ Making AVCHD DVDs == Latest changes (modified 29th May 2018 14:43.)
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