Since Doctor Who began broadcasting new episodes in 2005, the BBC has been releasing DVDs and later Blu-rays of the series. For an American collector, purchasing these episodes can get very expensive, even from amazon.com. I purchased all the "classic" Doctor Who serials in their "pure", U.K. Region 2 format from amazon.co.uk.
For purposes of this article, 25i=50i and 29.97i = 59.94i. 25i is 25 interlaced frames, 50i is 50 interlaced fields, 29.97i is 29.97 interlaced frames and 59.94i is 59.94 interlaced fields. An interlaced field breaks up a complete frame into odd and even scanlines.
The Ninth Doctor, series 1, and the Tenth Doctor, series 2-4 and the Tenth Doctor Special The Day of the Doctor were originally recorded and broadcast using the standard standard definition widescreen PAL format 576/25i. They used a post-processing effect to give the program a more film-like progressive scan (25p) quality.
The later Tenth Doctor Specials, starting with Planet of the Dead and all the Eleventh Doctor stories, were recorded in high definition 1080/25p and usually broadcast on BBC HD or BBC One HD in the 1080/25i format. The fact that the interlaced format is used should make no difference in the picture quality. 25 frames splits evenly into 50 odd and even fields. Progressive, segmented Frame treats 25i material as 25p material for all intents and purposes.
The Day of the Doctor, the 50th Anniversary Special, was recorded in 3D. It was broadcast in 3D on the BBC Red Button HD channel. According to the instructions, the user needed to set his TV to Side-by-Side mode to watch the broadcast. There is a Top-and-bottom mode, but side-by-side officially supports 1080 @ 50Hz. Both methods are designed to squeeze the 1080 3D signal in the same bandwidth allocated for a 1080i broadcast channel. In side by side, the left and right 3D frames are combined horizontally, and when displayed, half the horizontal resolution is lost. In top and bottom, the left frame is stacked on the right frame, and when displayed half the vertical resolution is lost. The left and right images are stretched back to the proper aspect ratio when displayed on a TV. See here for more detail : http://www.cnet.com/news/how-3d-content-works-blu-ray-vs-broadcast/
This leads me to discuss the two 3DTV systems on the market today, Active 3D and Passive 3D. Active 3D uses expensive shutter glasses to block the eye from seeing the wrong frame. Passive 3D uses inexpensive polarized glasses to filter out the light emitted from the wrong frame. Passive 3D is also used in theaters because the glasses are practically given away. However, due to the way that passive 3D works, (left frame = odd lines displayed, right frame = even lines displayed) the effective resolution of each 3D frame is 1920x540, so the top and bottom mode is typically not used for 1080. Active 3D has its issues as well, including crosstalk, headaches, expensive, battery powered and uncomfortable glasses and people trying to wear those glasses over eyeglasses. See here for more detail : http://www.cnet.com/news/active-3d-vs-passive-3d-whats-better/
DVD supports 576/25i for PAL countries and 480/29.97i for NTSC countries. When released on PAL DVD, the BBC could transfer the show very easily since DVD supported 576/25i. When released on NTSC, 576/25i material has to be converted to 480/29.97i. Lines must be dropped or blended and detail is lost because of the reduction in resolution. Because of the increased NTSC field rate, certain fields have to be repeated or blended and this can create stutter in motion, especially during camera movement.
Blu-ray supports 1080/24p and 1080/23.976p, 1080/29.97i and 1080/25i. It does not natively support 1080/25p, although many players may be able to play this format. Any product advertised that uses a progressive frame format will be in 1080/24p or 1080/23.976p, usually the latter.
Blu-Ray 3D only supports 1080/23.976p x 2. However, its 3D format uses frame packing/stacking, and technically the resolution is 1920x2025 @ 23.976p. (45 blanking lines separate the left and the right imgaes) With frame packing, the full 1920x1080 frame for each eye is transmitted, so the full horizontal and vertical resolution of each 3D frame is preserved. Whether your TV will show the full resolution depends on the type of 3D used.
(There are also 720 resolutions in 2D and 3D Blu-ray, but Doctor Who always uses 1080 resolution on Blu-rays.)
Doctor Who on DVD
For Classic and New Series Doctor Who, up and until they started recording in HD, everything that has been released on DVD is essentially the best the program can look without an upconversion. In this case, Region 2 is the way to go. Not only do you get the series in its native format, you can always buy the discs cheaper. Series 1-4 is contained in a very reasonably priced DVD boxset. Additionally, certain issues with music rights, which cause edits for overseas releases, are almost never an issue with Region 2 discs. All Region 2 DVDs are region locked to Region 2 (and most also support Region 4), however, bypassing region encoding on DVDs is easy enough.
Doctor Who on Blu-ray
For Blu-ray, the issue is more complex. First let's deal with the region encoding issue.
The good news : All Doctor Who U.K. Blu-rays, with a few exceptions, are region free, except for :
The bad news : The upconversions of Series 1-4 in the The Complete Serials 1-7 U.K. Boxset are Region B locked. The separate releases of these Series in Australia may also be Region B locked. I do not yet know whether The Day of the Doctor or The Time of the Doctor U.K. releases are Region B locked.
Second, we must address the slowdown that comes when converting 25i/p material to 23.976p material, as is the case for Series 1-4 and The Next Doctor on Blu-ray. The episodes run roughly 4.1% longer when converted. The action will seem a little slower, but only by comparison. The pitch of the audio, including voices are also affected by up to half a semitone. The audio is pitch shifted during the conversion software. I have read that the scrolling credits will appear markedly less smooth. The slowdown does not occur with the DVD releases of this material. While this may add something close to two minutes to each episode, it is nowhere near as much as an issue as for example in Rainer Werner Fassbinder's Berlin Alexanderplatz, a fifteen hour long film made for PAL TV. See here for a discussion of the choices The Criterion Collection made to the frame rate of Fassbinder's epic masterpiece when releasing it on DVD in the U.S. : http://www.criterion.com/current/posts/732-reality-at-25-24-frames-per-second
However, the video quality improves generally due to more efficient encoding techniques of Blu-ray, the use of more recent and improved mastering software and professional upscaling.
However, as the Series 1-7 box set was a limited edition in the U.S., and it goes in and out of print. The U.K. doesn't have this problem. Amazon is advertising it for $349.98 while Amazon UK has it for £165.24. Assuming £1=$1.70, the U.K. version is cheaper.
The Complete Seasons 5, 6 and 7, all use 1080/25i, so I am sure that they will not suffer from slowdown. I also understand that the bare-bones story releases (the discs they release before the Complete releases) also do not have this problem. Neither should The Time of the Doctor Blu-ray. The Complete Season 5 is missing the Next Time trailers and has a wrong version of the Children in Need special.
The Complete Tenth Doctor Specials, regardless of region, use 1080/29.97i. HDTVs sold in formerly PAL countries apparently do not have a problem with the NTSC field/frame rates. I have read the results are a bit mixed, and should not be an issue with the DVDs, but typically resolution trumps frame rate.
The Complete Serials 1-7 U.K. Boxset contains the previous standalone releases of The Complete Tenth Doctor Specials and The Complete Seasons 5, 6 and 7. Series 1-4 is new. The Complete Serials 1-7 U.S. Boxset contains newer masters of The Complete Tenth Doctor Specials and The Complete Seasons 5, 6 and 7 and everything is in 23.976p.
Unfortunately, The Day of the Doctor Blu-ray will suffer from slowdown regardless of the country it is released in. This is because the Blu-ray 3D spec only supports 1080 lines at 23.976 frames per second. The accompanying DVD in the U.K. release should not suffer from this problem, but then it is not in its native 3D format either.
Spearhead from Space, the only Classic Doctor Who story that has been released on Blu-ray, was released because it was shot solely on 16mm film. The BBC retained the film and thus could scan it in HD and transfer it to Blu-ray with a noticeable upgrade in quality over DVD. Any other classic Who stories would have to be upconverted from 756/25i and thus would not be worth the expense. On the Region B Blu-ray, the disc is encoded in 1080 /25i, while for Region A, the disc is using 1080/23.976p.
On a closely related matter, An Adventure in Space and Time, which is only currently available as a Blu-ray in the U.S. and Canada, must suffer from the slowdown. If you want to see it in its native speed but not its native resolution, you can buy the U.K. DVD. The Blu-ray is not region encoded, so I am sure that it will be a relatively popular reverse-import.
Notwithstanding the slowdown issue, buying Blu-ray from Amazon.co.uk. is still cheaper than buying anything in the U.S. in most instances. If you care more about consistency and picture quality, buy the U.S. Complete Series 1-7. If you care more about speed issues, then buy the U.K. DVD Series 1-4 and the Complete Specials and Series 5-7.