Posts Tagged 'mp3'

Impedance, Hi-Fi, Headphones and Guitars

Impedance is an important part of audio technology, all the way from plugging your guitar into your amp, your amp into your cab, the cabinet microphone into the preamp and all the downstream connections, both digital and analogue, right the way to your listening experience, from speakers to plugging headphones into your mp3 player. However, how much do we know as producers and music listeners, and how much do we need to?

In all actuality, most of us don’t really need to know that much about it, previous generations of telecom, audio and electrical engineers have addressed a lot of the issues for us and, generally, if we plug something into something else, everything has been designed so that it will just work. There may, however, be a couple of things you may want to consider…

What is impedance?

For those of an electrical persuasion, impedance (Z) represents the total resistance of a particular circuit including, not just the resistors, but also the resistive contributions of other components such as capacitors and inductors. Like resistance, it is measured in Ohms. Voltage (V), current (I) and resistance (R) are related following Ohm’s law: V=IR; power (P) is the product of the voltage and the current so P=VI or P=V^2/R if we do a bit of substituting and rearranging. From that, we can see that resistance/impedance can affect our power output. And that is probably as far as we need to go here.

What does this mean for my hi-fi?

Most hi-fi separates equipment that connects using RCA/phono connectors has been designed with appropriate output and input impedances, so there isn’t usually anything to worry about. However, there might be one area where it pays to think about impedance: you can generally ignore marketing copy that tells you to spend a fortune on hugely expensive speaker cables, for most purposes, using thick, 2-core mains cable will be fine. What you are aiming to do is to reduce the resistance of the cable to ensure maximum power transfer to your speakers. Resistance is proportional to the length of the wire and inversely proportional to the cross-sectional area, the thicker the wire, the lower the resistance. The cheap doorbell cables that come with a lot of mini systems is often quite high resistance, and you lose a lot of the power as heat. Most hi-fi separates equipment that connects using RCA/phono connectors has been designed with appropriate output and input impedances, so there isn’t usually anything to worry about.

What does this mean for my headphones?

Something you may have seen in the specifications list while headphone shopping is the impedance rating. Generally speaking, higher impedance headphones will be quieter for a given volume setting on your device than lower impedance headphones. However, you do need to consider the efficiency of the driver (you may see it in the specifications as a number followed by dB/mW). Generally though, if you have your mp3 player at 100% volume with the included earbuds (possible around 16 or 32 Ohms, you’ll probably deafen yourself, but plug in some 250 Ohm ‘phones, and you may find you’re able to listen quite comfortably. You’ll also draw less current (as V=IR), so the battery might last longer, but remember you’re trading off volume: you don’t get something for nothing.

Higher impedance headphones are often used for more ‘professional’ audio applications. The increased number of turns of wire can lead to a better ‘motor system’ in dynamic headphones, improving the general sound; also, the reduced current draw can reduce distortion in the amplifier. So, for ‘pro’ applications when you’re plugged into a powerful amp, high impedance is the way to go, for portable devices go with low impedance if you like volume.

What does this mean for my guitar?

If you’re recording your guitar DI, then make sure that you are plugged into a high-impedance (Hi-Z) input. A good rule of thumb is an input impedance of more than ten times the output impedance. Guitar pickups usually quote the DC resistance figure (there is a lot of wire when you start to add a lot of turns!) and are also inductive. Direct inputs for guitars are often rated to 1 mega ohm and higher to present a large load. Additionally, make sure that you use a reasonable quality guitar cable, as some cables can exhibit sufficiently high capacitance to cause problems. And what are these problems? A loss of high end frequencies (the output impedance and cable capacitance act as a high-cut filter) and a reduction in the guitarist’s beloved sustain.

The other area the guitarist needs to pay attention is when plugging a speaker cabinet into their valve amplifier. Valve amplifiers can be very sensitive to inappropriate loads on the output transformer. Without an appropriate load on the secondary coil, the energy in the magnetic field of the primary coil can’t be completely transferred, collapses back and sends that energy back to the valve as a ‘flyback’ voltage. This is usually not good. For that reason, use appropriately rated amps and cabs (a number of amps have different rated outputs for connecting to different cabs) and contact the manufacturer if you are unsure.

Beauty in Music’s Physical Form

I’m not a neophile, I’ll quite happily admit that; I do not immediately jump on board the latest craze or gadget just because it’s new. However, I wouldn’t say I was especially big on nostalgia either. I don’t automatically think that something is better just because they don’t make it anymore. I just like things to work and I don’t see the point of change for change’s sake. For example, I like TVs with buttons on them so I can use them when I’ve lost the remote, but I like being able to take thousands of pictures on holiday, store them on one tiny card and only print out the ones that aren’t of my thumb.

All of which brings me somewhat inelegantly onto the subject of digital music downloads.

Digital music delivery is huge, I don’t think you can deny that, but is it an improvement over where we were before? It has certainly made music more available, it’s now incredibly straightforward for a new unsigned artist to get tracks out to people, but I think that’s a subject for another post. I’m more interested in the concept of buying something that doesn’t really exist in a physical form. In order to discuss the present case of music downloads though, I think it’s best to start with the past…

With each new format of sound delivery, there has traditionally been an increase in quality, from wax cylinders, to vinyl discs, to the cassette to the CD. Now, I know that vinyl is actually better in terms of certain specifications than the digital constraints of CD, but that’s a whole other post again… Generally though, each successive format has improved our listening experience. Then, in 1999, along came the super-audio CD (SACD). Using a 1-bit high frequency technology, the SACD offered audio specifications approximately equivalent to 24 bit 96 kHz recording, with better frequency response and dynamic range than the original CD. An excellent review on the science behind the quality of SACD can be found in Hugh Robjohns’ interesting Sound on Sound article.

While some hi-fi audiophiles picked up SACD players, it just didn’t catch on the same way CD did; the same for DVD-Audio. It appears that the CD is the plateau of consumer quality, a point above which it wasn’t worthwhile climbing. And it makes sense. What percentage of the world’s music listeners even own a hi-fi separates system these days, let alone have their chair neatly positioned in the stereo sweet spot and some acoustic treatment in their listening room?

So, the next step beyond the CD: a step backwards in quality. Limited bandwidth compressed audio: the all-conquering mp3. Now, this brings me back to the first paragraph, I think the mp3 is a wonderful thing. I can carry thousands of songs in my pocket (I sometimes miss my CD Walkman days though, but that’s also another post…) and I don’t really notice the loss of quality as I’m stood in a bus shelter, next to a main road, when it’s raining and listening through ear buds.

The thing is though: I rarely buy mp3s. There are the occasional tracks, maybe a random tune from an album where I don’t want to buy the full thing, but that’s really it. Virtually all my mp3s are ripped from the CDs I buy. The reason (apart from the whole quality thing): I like the physical interaction you get when you buy a record. Be it CD or vinyl, it’s a wonderful thing to look at the cover art, read the liner notes, it becomes more of a ritual, like grinding the beans for your morning coffee. Taking the record from the sleeve, sitting it on the platter, the thunk, hssssss, as the needle finds its groove… wonderful. And to come back to the visual art of vinyl and CDs, some of it is superb purely on that level, from beautiful to iconic and back again, records can be very pretty things… and I’m not just talking about my 12” Kylie Minogue Slow picture disc.

Back in my teenage years of limited pocket money, as opposed to now with limited disposable income but the benefits of credit, I could perhaps afford a CD every two to three weeks. I would end up knowing that album inside and out, I could draw the cover from memory (not very well, art was never really my thing), tell you who produced and engineered it, where it was recorded and even give you a potted list of the people in the thank you list.

I still get that same buzz today when the post delivers that tell-tale cardboard sleeve containing the latest vinyl purchase. The Hidden Orchestra limited double 12” on the clear/marbled vinyl and my signed Jo Mango 10” (she says I’m a wee star!) remain wondrous things to behold before you even listen to them, along with my complete set of the Journal of Popular Noise 7”s, an original 45 of The Velvelettes Needle in a Haystack and my early Amy Winehouse CD promos, the list goes on, but I love them all dearly. I just don’t get that same feeling with a mouse click and a little download progress bar somehow…


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