Supergris skrev:Du kan sagtens bruge almindelige phonokabler til alle farver, da kablerne er samme type. Farverne er blot for at gøre det enklere, og så er de som regel dyrere...
Nej kablerne er typisk ikke samme type.
Videokabler og S/PDIF kabler har en impedans på 75 ohm, de har almindelige audiokabler normalt ikke, hvilket resulterer i delvis signal-refleksion og dermed tab af signalstyrke. Audiokabler har typisk en impedans på 35-50 ohm og er derfor ikke særligt velengent til video som altid er impedanstilpasset til 75-ohm.
Til audio er kabelimpedansen ligegyldig, da indgangene er højimpedante og udgangene lavimpedante og signalet er meget lavfrekvent.
Men til S/PDIF og video som er meget mere højfrekvent, er både ind- og udgange og kabler impedanstilpasset til 75 ohm.
Derfor kan man sagtens bruge video- og S/PDIF-kabler til audio, men ikke nødvendigvis altid omvendt.
http://www.audioholics.com/education/ca ... tive-guide
1.0 Engineering 75-ohm Interconnects
Unlike audio cables, which only conduct low frequency data on the order of 20Hz to 20,000Hz, video cable must transmit higher frequencies up to levels of around 8MHz to 10MHz for NTSC and for over 35MHz for HDTV. At these frequencies, there are a number of factors that can create interference, signal loss, and signal degradation, all of which can be identified and minimized by applying fundamental electrical engineering principles. This article is intended to discuss these basic engineering principles, provide information on various manufacturing techniques of component video cables and coaxial cables, and explain the different types of component video cables on the market. There are a number of concepts and misconceptions about component video cables that will be addressed in this article, many of which will be proved or disproved mathematically. As you read this article, it is not necessary to focus on the calculations as much as the conclusions that are drawn by them.
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One method of considering impedance is being comprised of reactive and resistance values and therefore related to resistance and capacitance. In a standard audio cable for example, the internal impedances are between 35-ohms and 50-ohms. Since they are designed to carry low frequencies (20Hz to 20kHz), resulting in much longer wavelengths than the length of the cables themselves, audio cables by their design, aren't concerned with characteristic impedance as much as lumped R,L,C parameters. However, if audio cables are used in place of component video cables, or poorly constructed component video cables are used that are not a true 75-ohm characteristic impedance, the lower impedance value of these cables may result in a partial signal reflection do to a mismatch in impedance, depending on the length of the cable. Detailed information on mismatched impedance and internal impedance are provided within the sections 2.2 and 3.0.
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2.3 Mismatched Impedance
Mismatched impedance is one of the most common and most frequently experienced sources of signal degradation. This phenomenon occurs when a high frequency bandwidth signal, designed to be matched to a characteristic impedance of 75-ohms, encounters different impedances through its signal path (IE. Transmission Line), usually on the order of 35-ohm or 50-ohm for Home Theater applications. It can occur in long runs of video cables ( > 1/10th the wavelength) that do not use true 75-ohm cables, or it can occur from the DVD player or the TV monitor, depending on the nature of their internal impedances. This mismatch can create a bounce back effect or reflection that results in a delay and/or loss of signal level for certain frequencies. Thus for component video cable, transmission line effects of any cable lengths beyond 3 meters (remember 1/10 of 30 meters) must be considered.
