Digital vs. Analog cable

The main difference is that there's only one channel, even though it's for stereo (or multichannel, resp.). Another difference is the theoretical 75 ohm wave resistance, instead of the 50 ohm optimum with analogue cables. That means the proportion between inductance and capacitance is different -- capacitance is relatively lower in a 75-ohm cable. You can achieve this e.g. by greater distance between hot conductor and ground/shielding.

i apologize in advance if this falls into the "duhh" category... but is it not true that the quality of digital cable matters less (still matters, of course), since the signal hasn't yet been converted to analogue?

From a purely technical stand-point, digital audio cables make the best analog cables. They have very low capacitaces...because they have to! In a digital system, there is little room for error. Digital cables are also built with higher frequencies in mind, so they should have no trouble with lower frequencies (such as audio) that are easier to transmit. If you are paranoid about signal integrity, digital is technically the way to go.

However, do not use analog cables as digital cables. Digital cables are built to a higher standard, and the higher capitance of analog cables can cause bit-errors.

Also, I would be very, very, very afraid of buying a digital cable from an exotic audiophile cable manufactuer, since they probably do not have technical expertise to manufactuer something with as stringent of requirements. Belden, Belkin, Acoustic Research or RatShack would make fine choices.

A digital signal is an analog signal. Depending on the coding scheme, you could have the classic square wave pattern or you could just have a high frequency analog signal. The analog signal works the same way as FM and AM radio. By controlling the frequency and amplititude of the carrier signals, you can encode a single or series of bits in one pulse. Really cool stuff and this is how they do it with stuff like wi-fi and wi-max. (They can actually transmit a unique 6 bit stream in a single pulse but I won't go into the technical stuff on how they do it).

Anyway, I think it's been pretty much covered. The operating frequencies of digital signals is much higher than audio, going up into the megahertz (VHF) for digital cable. The result being that digital cables are designed for more stringent parameters to reduce attenuation, dispersion, and induced noise. In addition, many digital interfaces use 75 Ohm cables (like digital cable) instead of 50 Ohm.

The ohmage of a cable is not its resistance, but its characteristic impedance. Impedance is a number that combines resistance, capacitance, and inductance and a perfectly loss-less cable still has a non-zero impedance. For a simple model, it represents the square root of the ratio of capacitance and inductance. So by itself, you cannot discern too much from it because you cannot determine what the absolute capacitances or inductances of the cable. What it does matter is in matching the cable and loads. Ideally, if you have a 75 ohm cable, then you want the load seen by the cable at its connections to be 75 ohms as well. If the load is not 75 ohms, then you will have a mismatch and reflections will occur. Reflections mean that part of the signal is bounced back along the cable instead of being totally transmitted (or absorbed) by the load. These reflections causes noise and jitter (delays the amount of time that it takes to tell if a digital bit is a one or a zero because of the noise).

Impedance mismatch is much more important with digital signals because of jitter (if it is applicable) and noise will create noticeable problems. With an analog signal, reflections are less important because of the large wavelength, speed of propagation, and length the cable. Simply put, the cable's signal is basically a DC signal because the wavelength is on the order of kilometers and any reflections will settle down in a very short time in comparison to the signal's frequency.

wow... thanks for that explanation. there are parts i don't understand, but i do get the gist of impedance matching.

so if a digital signal is an analog signal traveling along the higher end of the spectrum... what actually happens in digital to analog conversion? and is it still correct to believe that in an ideal world, eliminating digital to analog conversion altogether (sending a wider-bandwidth amplified digital signal to the speaker) would be preferable?

i hope i'm not threadjacking... but it seems germane to the discussion of analog vs. digital cable.

Digital to analog conversion takes the digital stream and reproduces the appropriate waveform. In the case of a CD's, the coding scheme is Pulse Code Modulation (PCM) which is where the bitstream gives the amplitude of the wave at 1/44,100 second intervals. The simplest way of converting it to an analog signal is using what is called a zero-order hold DAC. In this scheme, you output a constant voltage that is equal to the amplitude of the wave over the interval of the sample. The end result is something that looks like a bunch of stairsteps rather than the smooth wave that you want. You've probably seen something like this represented in marketing stuff about upsampling and whatnot. They usually have a picture showing how much smoother the stepped wave looks like if you use 96KHz/24bit (Creative had this on one of their Audigy packaging). This is not completely true because this is not the entire conversion process. The stairstep only approximates the signal, so to fully regenerate the analog signal, you pass the stairstep wave through a filter. If we had an ideal filter, we could theoretically perfectly reproduce the analog signal. There are other ways of doing the conversion (like using interpolation) but that is the simplest method.

Another thing to keep in mind is that the classic square wave of a digital signal (high-->low-->low-->high...etc) is still a combination of an infinite number of frequencies. The higher the range of frequencies that your signal generator can produce, the faster you can make the signal go from high to low and vice-versa and thus the faster you can make your clock. So even in this case you need to be mindful of how the higher frequencies behave in a cable.

As to removing the DAC entirely, I cannot say. In an ideal world, we could realize the mathematics of the digital to analog conversions and the process would be ideal anyway. Without a DAC, then you still have the problem of taking what is in most ways a symbolic representation of the analog wave and converting that into the soundwave that you want. If it is possible to go directly from digital to the mechanical wave it is something that I do not know about.

the new pure path digital amps supposedly don't convert the signal to analog in the traditional sense.

from what i understand (which is probably not at all), they amplify the PCM signal, then convert it to a PWM signal that's delivered to the speakers. like i said, my understanding of this is minimal.

That answers a few questions, but raises so many new ones.

The majority of digital amps do this: PCM --> Analog --> PWM (this is what tripath does)

They reason they do this is because it is cheaper and has lower distortion with today's technology. Doing a pure PCM --> PWM creates more trouble than its worth.

May I say that I love the Adam Smith quote? Economics forever!

<--- Business Major