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The Plextor ConvertX, even at an early stage in its Linux support cycle, is a legitimate alternative to the Hauppage video capture cards that so many Linux users are using these days. Last March, Plextor announced the availability of a Linux software development kit for two of their personal video recorders: the ConvertX PVR model PX-TV402U and the ConvertX PX-M402U. I chose to evaluate the ConvertX PVR-PX-TV402U product because I wanted to find a suitable piece of hardware to use in a system that functions as a personal video recorder.
Setting up the hardware
My test system included the following:
You can download the Linux SDK for PX-M402 and PX-TV402 for free from Plextor's Web site. Once you unzip and extract the tar.gz file, you have several standard source code files, including a Makefile and a README file. I recommend that you first read the README file because it contains important information about what you'll need in order to successfully use this hardware.
You'll also need hotplug, hotplug firmware, and USB support compiled into your kernel.
The video capture quality was very good, but it depended upon the source signal. If your source signal is poor with static, you can't expect the result to be anything better, and it may even be worse. However, if you have a clear video signal, you should obtain good, clear captures.
On my test system, the CPU and MEM usage (according to the
top command) never went above 3% and 2% respectively. The resolution and format appeared to make no difference in the resource usage figures. As you may know, when you try to capture at a resolution of 720x480 using a cheap TV tuner card, you get dropped frames and higher system resource usage. That wasn't the case with the Plextor card. This is a good indicator of its potential in software setups like MythTV, which require the ability to record one show and play back another simultaneously without maxing out the system.
|The Plextor ConvertX PVR|
Some may wonder if a USB 2.0 port is necessary. I was able to perform captures with a USB 1.1 port, but I sometimes had problems playing back MPEG4-formatted captures with the 1.1 port. These problems weren't present when I used a USB 2.0 port. I never experienced any problems capturing MPEG2-DVD video at 720x480. At certain resolutions and formats, a USB 1.1 port may be sufficient. For some reason, with a USB 1.1 port, I always had to unplug the power and plug it back in on the unit for the system to recognize it and load the drivers. With the USB 2.0 port, I didn't have this problem, and the drivers were loaded at boot time.
The Linux SDK provides a command line program called gorecord that gives a good idea of the unit's capabilities and features. For instance, you can set the format you want to capture in (e.g. MPEG2, MPEG2-DVD, MPEG4, MPEG1 or MJPEG) as well as a wide variety of other preferences. Since the TV402U comes with an integrated TV tuner, you can tell it what channel you want to capture. Or if you want to use the S-Video or composite input, there is a setting for that, too. A full list of options can be seen by typing
gorecord --help from the command line.
I found that a bit rate setting of 1500 to 1750 kilobits per second provided suitable video quality for 720x480 MPEG2-DVD captures. Going beyond that didn't appear to make much difference in terms of video quality. Something to keep in mind is that the
-bitrate option is for the video bit rate. The audio bit rate will always be 1536 kilobits per second, so to get the total bit rate, you have to add on the 1536.
Even though the gorecord application sports a tremendous amount of flexibility and features that allow you to exploit every aspect of this hardware, Nathan Lutchansky, the author of the Plextor Linux driver, told me in an email, "The gorecord application serves as example code only and does not aim for maximum compatibility or user-friendliness. It also does not perform A/V synchronization, so the audio and video will usually get out-of-sync after 30 to 45 minutes."
The A/V sync mismatch doesn't have to be much in order to make things really ugly. On a few tests, I found that some videos needed only an adjustment of 0.12 seconds from within Xine in order to bring the audio and video back into adjustment. That may not sound like much, but it can be very noticeable on certain types of video. The best remedy for this problem is to record with MythTV, which natively supports the Plextor ConvertX.
|A broadcast on MythTV|
|The same broadcast as recorded by the ConvertX|
Working with video files
The MJPEG files that gorecord produces are compatible with the MJPEG Tools suite. Although the programs in MJPEG Tools can use software to encode to a DVD video file, the process is very CPU-intensive. No one wants to do this when he has an expensive piece of hardware that's capable of capturing to a compressed format.
If you need a piece of hardware to use from within MythTV, you should consider the Plextor ConvertX PX-TV402U. The quality of the recorded video is excellent, and the system resource usage of the unit is extremely low, which means you get excellent performance when using it from within MythTV. The unit is also a welcome possibility for video capture on laptops since very few options currently exist.
Plextor has added life to Linux by offering full software support for the Plextor ConvertX in terms of open source drivers. You just can't beat that. Every piece of hardware that is supported in this way gives users yet another reason to stay with Linux.
|OS Support||GNU/Linux, Windows 2000/XP|
|Market||Video recording enthusiasts|
|Price (retail)||U.S. $200|
|Product Web site||Click here|