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4. PCI, USB or FireWire?

Which bus should I choose?

Once the data from the sound card has been coverted to the correct format it needs to be channeled into the heart of the computer, ultimately reaching the program (ie, the audio sequencer). To achieve this, the continuous audio data stream is sent in small bites via a so-called ''data bus'' to the memory of the computer. A bus is to digital audio as a cable is to analog audio - a transmission path from A to B.

The currently relevant bus types for audio applications are:

  • PCI/PCI-X (Peripheral Component Interconnect)

A parallel data bus where multiple lines move the bits into the computer simultaneously (PCI 32 lines, PCI-X 64 lines). The PCI bus is part of the computer itself, and offers a connection directly to the motherboard in the form of a slot for a PCB (printed circuit board).

PCI/PCI-X (Peripheral Component Interconnect)
  • PCIe/PCI-E (PCI express, not to be confused with PCI-X)

Although similar to PCI in that it's placed directly on the motherboard, they are fundamentally different in that PCIe is a serial bus. A single lane is about twice as fast as regular PCI, and PCIe can have up to 32 lanes moving up to 8GB of data per second.

PCIe/PCI-E (PCI “express”, not to be confused with PCI-X)

  • FireWire/IEEE-1394 (1394 is the designation of the Institute of Electrical and Electronics Engineers).

FireWire/IEEE-1394 is the best example of a serial bus. The data is sent one bit behind the other over a single pair of lines. This allows a relatively long cable to be connected between the computer's port and the external audio interface.

FireWire/IEEE-1394 (“1394“ is the designation of the Institute of Electrical and Electronics Engineers).
  • USB 1.0/2.0/3.0 (Universal Serial Bus)

The other popular serial bus, favoured by Intel and Microsoft. While FireWire devices can communicate independently among themselves, USB uses a kind of tree structure hosted by the computer.

USB 1.0/2.0/3.0 (Universal Serial Bus)
  • Thunderbolt

This interface was developed by Intel in collaboration with Apple. Technically speaking it's a cross between DisplayPort and PCI Express. Currently there are only a few interfaces that are equipped with Thunderbolt.

Thunderbolt
  • CardBus/ExpressCard/PCMCIA (Personal Computer Memory Card International Association)

A laptop expansion slot which corresponds to a parallel PCI bus, only in a substantially more compact version and with the possibility of inserting the card from outside the computer.

CardBus/ExpressCard/PCMCIA (Personal Computer Memory Card International Association)

Within the individual bus families there are variants with different speeds:

  • FireWire 400 - up to 400 megabits per second (Mbit/s)
  • FireWire 800 (1394b) – up to 800Mbit/s
  • USB 1.1 – up to 12Mbit/s
  • USB 2.0 - up to 480Mbit/s
  • PCI (32bit/33Mhz) - 133MByte/s (1066Mbit/s)
  • PCI-X (64bit/133MHz) - 1066MByte/s (8528Mbit/s)
  • PCIe x1 (1 Lane) - 250MByte/s (2000Mbit/s)
  • PCIe x16 (16 Lanes) - 4000MByte/s (32,000Mbit/s)
"The bare comparison of data transfer rates"

The graph should be viewed with some caution as it only represents theoretical maximums. In the real world, the speeds actually attainable can be much slower, but it is still helpful for a rough comparison. The data transfer rate depends on how many audio channels can be transmitted simultaneously.

Unbalanced, balanced or auto-balanced?

All single channel electrical signals are unbalanced and travel down a single piece of wire (plus ground). A balanced line uses two wires, with the original signal going down one and a mirror image going down the other which has been phase reversed. When the phase is reversed back at the other end and the two signals are combined, any interference picked up in the cable is cancelled out. Auto-balanced cables switch between being balanced or unbalanced depending on what’s attached.

What is stacking?

Combining two or more interfaces of the same make and type to expand the number of connections available. Drivers must support this in order for it to work, and not all do. Audio interfaces from different manufacturers cannot be stacked in this fashion.

Which level, -10dB or +4dB?

Ultimately it depends on the equipment. +4dB is “hotter” and so has more level than -10dB, and on output this could cause distortion through the amplifier. On the other hand –10dB can be too quiet and waste some of the resolution of your dynamic processors. Most interfaces can be switched between the two, so that you can find the idea level for your particular environment.

What is Phantom Powering?

This is usually 48V of power that is required to run a condenser microphone. It is sent back down the same cable as the audio signal arrives on, requiring no separate connection, hence ‘phantom’.

What’s the difference between Coaxial and Optical S/PDIF?

Coax cables are highly insulated electrical cables, where the signal line runs down the centre of the shielding – antenna cables are coaxial. Optical cables use light that is transferred down a fibre optic cable. Both cables are suitable for digital signal transmissions, although optical is less prone to interference from electrical fields. Coax and optical are not directly compatible with each other and require a conversion box to switch between the two formats.

Does 96kHz sound better?

Although in theory we cannot hear the extra information that is recorded at 96kHz or even higher, it often does sound better, primarily because it allows ‘anti-aliasing’ filters to be placed at such a high frequency that they have far less detrimental effects on the audio spectrum. There are many other factors that affect audio quality though, so it can only be said as a rule that 96kHz generally sounds a little better than 48kHz on the same audio interface. 96kHz also generates double the amount of data, and so requires the computer to work much harder to cope with the information, and needs double the disk space to store it.

Is the quality of converters important?

Yes, but as with many things, the law of diminishing returns comes into play – although as a general rule you will get better quality if you pay more, paying double doesn’t double the quality. Unless you have highly trained ears, you may find it hard to detect any difference as you go further and further up the price range.

How high should the signal-to-noise ratio be?

Naturally, in principle, as high as possible. However, the higher you go, the harder it becomes to notice the difference. The maximum SNR for CD is 96dB and that’s pretty quiet. To make use of any SNR above 96dB you’d need to be working in 24bit resolution, which is one of the advantages of working at resolutions higher than 16bit.

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