2012-10-13 HDMI線 平貴全無分別

MichaelNg

簡單咁講, 佢只係傳送 010101 信號. 一係有, 一係無. 無分好壞.

Right now you can buy a 6-foot-long HDMI cable for $3.50. Or $19.99. Or $99.99. Or $699.99. Salespeople, retailers, and especially cable manufacturers want you to believe that you'll get better picture and sound quality with a more expensive HDMI cable.
They're lying. You see, there's lots of money in cables. Your money.
Dozens of reputable and disreputable companies market HDMI cables, and many outright lie to consumers about the "advantages" of their product.  
Worse, the profit potential of cables is so great, every retailer pushes high-end HDMI cables in the hopes of duping the buyer into spending tens, if not hundreds, of dollars more than necessary.
Here's the deal: expensive HDMI cables offer no difference in picture quality over cheap HDMI cables. CNET has mentioned this before, but here's the science of why.
The signal
The first thing to understand is what's transmitted over the cable in the first place. HDMI uses Transition Minimized Differential Signaling, or TMDS.
TMDS has two basic aspects. The first is that the ones and zeros at the source (a Blu-ray player or HD cable/satellite box) are not exactly the ones and zeros your TV uses to create a picture -- at least, not in exactly the same order. Before sending the signal out via the HDMI output, the ones and zeros are rearranged to minimize how many transitions there are. So instead of 10101010, the transmission may look like 11110000. If you really like math, how it does this is cool, but it's not really important to understanding the concept as a whole.

MichaelNg

ven though this conversion is weird, it makes it much more likely that the data transmitted can be rebuilt on the other end (as in, at the display).
The second part of TMDS (the DS part) is the HDMI cable itself. Each HDMI cable is actually multiple small, copper wires. Two versions of the data are sent over different wires. One of these is out of phase with the "real" signal. The TV receives all the data, puts the out-of-phase signal back in phase, then compares it to the "real" signal. Any noise picked up along the way will now be out of phase, and as such it is effectively negated and ignored.
If you're an audio person, this is similar to how balanced (XLR) cables work.
TMDS works really well, allowing for short cables and fairly long cables to carry what is a pretty intense amount of data. It also means you can have inexpensive cables that work just as well as expensive ones.
　
More important to our discussion, it means that when something goes wrong, it goes really wrong. It's often said that with an HDMI signal, you either get everything and it's perfect, or it isn't perfect and you get nothing. In fact, I've said this. If you're getting an image that looks correct, and there are no dropouts in the audio or video, then you're getting everything that's being sent. If the cable is faulty, or it's a really long run with an under-built cable, most of the time you'll just get nothing. No picture at all.