Recording Digital Cable
Recording digital cable versus analog cable should, in theory, be a simpler matter than recording its analog counterpart, due to the fact that unlike analog television, digital television need not undergo analog to digital conversion and encoding to be digitally stored on a computer. Instead, the digital stream is simply "downloaded" as is in the form of a transport stream. In practice, however, recording digital cable is more complex due to industry imposed encryption; cable companies often encrypt the channels provided via their cable service such that only rented set top boxes (or other means such as cablecard) are capable of receiving and "descrambling" the channels for viewing.
To that end, two of the three options for recording digital cable solely depend on the encryption status of your cable service. These two options are to use a QAM tuner card to tune the digital channels or to use firewire output from a cable company provided cable box, and they are the sole means of viewing digital cable channels in their unaltered digital form, as well as the only means to receive high definition television provided through digital cable. The only option that works regardless of encryption status is the analog tuner card option, but this method does not preserve the digital quality of the incoming video and hence can produce serious degradation of the quality of the video depending on a number of factors. Read on for a break down of these three options.
Option 1: Using a QAM Tuner Card
In the US, digital cable is generally transmitted via quadrature amplitude modulation or QAM, a means of transmitting digital data such as digital video streams. These streams, if unencrypted, can be tuned via a QAM tuner card such as the Avermedia A180 or Kworld XXXXXX. These cards simply "download" the digital video streams straight from the cable to your hard drive in their original form, hence preserving their original quality. They support both standard definition digital television and high definition digital television.
This method is the most reliable means of recording digital cable, assuming your QAM channels are not encrypted, as it does not rely on any outside hardware from the cable company which avoids the support problems of firewire set top boxes as stated below. Unfortunately, QAM channels are almost always provided encrypted, so this method does not work for most. The only exception is the free to air (FTA) high definiton channels (such as ABC, CBS, NBC, etc..) that cable companies by law must provide in the clear, though that's hardly a consolation as these channels are provided for free over the air via the ATSC standard.
To check if your channels are provided in the clear via QAM, you must attempt to tune them either with a QAM tuner card in a PC, or with a newer television set that has a built in QAM tuner. Alternatively, you can contact your local cable company, but that may prove to be a frustrating experience. It's also important to note that cable companies can change their encryption status when they please, so while your channels may be unencrypted today they might not be tomorrow. This isn't too common, but it is possible and definitely something to be aware of.
Option 2: Using Firewire Output from a Set Top Box
Using this method, a set top box rented from the cable company is used access your digital cable channels and then pass through the unencrypted digital video streams to your computer via firewire. This method also preserves the digital quality of the video streams, and it also has the added benefit of being able to work when the QAM channels are encrypted. In this case, the set top box essentially decrypts the video being provided by the cable company and then passes it to the computer in an unencrypted form via a firewire interface. Myth either changes channels via the firewire interface itself, or if that function is not supported on the set top box in question, an IR blaster can be used, and then the output from the firewire is simply "downloaded" to your system in its original transport stream.
The problem is that in addition to QAM encryption, digital cable channels can also be encrypted such that they are not able to be passed through via the firewire. This is known as 5C encryption, and channels encrypted with it cannot be accessed via firewire. 5C encryption varies by local franchise, and can often be seemingly quite random where some channels are encrypted but others are not without apparent reason. Some franchises such as some in the Seattle and Portland, OR areas provide nearly all of their channels without 5C encryption, and at the very least it is much more common than unencrypted QAM channels, so it is often the best way to record digital cable.
The downside is that using this method you must rely upon a rented piece of hardware from your cable company that may be subject to change. And because of the variety of cable boxes on the market, it is impossible to support all of them, and at this point Myth's firewire support should be considered young. It works rock solid for some, but for many it is less then 100 percent reliable and can require considerable tweaking to get working in such a manner. This support is improving considerably with each release, however. Also, as cable companies are only required by law to provide cable boxes with firewire output for high definition subscriptions, you may have to pay extra to get such a box as most companies charge extra for high definition enabled cable service.
To check if your channels are provided in the clear via firewire, you must attempt to tune them via the firewire interface or use a cable box that has a diagnostic menu that is capable (such as many Motorola set top boxes) of giving the 5C status of individual channels. Alternatively you can contact your local cable company, but as mentioned earlier, that may prove to be a frustrating experience. You can ask others who may live in the same area as you what they have found with their firewire setup, but as individual franchises can have different 5C setups, this should not be taken to be infallible. It's also important to note that cable companies can change their encryption status when they please, so while your channels may be unencrypted today they might not be tomorrow. This isn't too common, but it is possible and definitely something to be aware of.
Option 3: Using an Analog Tuner Card, Set Top Box, and an IR Blaster
With this method, a set top box rented from the cable company is used access your digital cable channels via its analog outputs, which is then recorded with an analog tuner card such as the PVR-150. Channels are changed via an IR blaster while the analog tuner card records the output of the box either through a coax, composite, or s-video input. Essentially, the analog tuner card records exactly what your TV would see if it was connected via an analog output of the set top box.
While this method is completely reliable and works whether or not your channels are encrypted, it presents, for some, a very serious problem. Because the digital video being received by your cable box must be converted to analog (to be transmitted via a coax, composite, or s-video output) then reconverted back to digital with an analog tuner card, the original digital quality of the video stream is degraded. This quality loss depends upon whether standard definition or high definition video is being viewed, and whether or not that video is being displayed on a standard or high definition screen.
With standard definition video the loss is not severe; it is definitely noticeable when being played back on a high definition television or computer monitor but not to the point of being unwatchable; however, on standard definition screens it is often indistinguishable from the digital source. With high definition video the loss is again generally indistinguishable from the source on standard definition televisions, but on high definition screens it is like night and day. The entire point of the enhanced resolution of high definition is lost, as the output from the set top box is down converted standard definition video. The bottom line is that you probably won't notice a big difference on standard definition televisions, but when using high definition televisions you will notice a decent difference in quality on standard definition video, and a huge difference in quality on high definition video.
As it is unlikely that cable companies will ever provide a completely reliable method means of recording encrypted digital cable channels on home brew HTPC systems, we must instead resort to other means. Ideally, at some point capture cards capable of capturing and encoding HDMI/DVI or component inputs will arrive. Right now, such products do exist but they are not capable of real time MPEG2 encoding, and to that end they produce uncompressed video of staggering size, and none are supported in linux.
Ultimately though, the only way to really fix this problem is to let cable companies know that their customers reserve the right to do as they please with the cable that they pay for, within the law, and to subsequently vote with our dollars.