USB Extender - A surprisingly useful tech

May 22, 2016 by Fu Yongwei in tech

USB cables are one of the most commonly used items in the computing world, regardless of whether you are an enthusiast or an IT average joe. You most likely have lost count on the number of USB cables you have owned. USB cables are ubiquitous as they provide essential features such as file / data transfer or device charge up. However, it is quite shocking that for a technology that has so much control towards your digital lifestyle, not many understand the specifications and limitations of this protocol. I myself have been bitten once in my own assumption towards this particular technology.

Similar to any data protocol, there is always a limitation in transmitting range, meaning there is always a maximum length you can run a cable before you lose signal integrity. In this case, the cable length should be 5m max for high-speed transfer and 3m for low-speed transfer.

You might wonder why there are USB cables more than 5m available in the market if the maximum length of USB cable is rated at 5m. There are even comments online saying that these cables work perfectly. I used to buy USB cables that are more than 5m for my recording / production works, mainly to punch in MIDI data into Apple's Logic Pro for sample triggering or instrument emulations. Using a USB cable for such applications is not uncommon, as this eliminates the need for a physical audio interface to connect the electronic instrument (e.g. Keyboard) to your computer (DAW). I adopted this approach as I often do not have access to a physical audio interface with MIDI I/O in most of my projects, thus using a USB cable to interface instrument with the DAW is a cheap and simple way to go about it.

Left: USB Type A to Type B cable, commonly used to interface between host and device (e.g. computer to printer)
Right: MIDI cable, commonly used to interface between electronic instrument to an audio interface

This approach worked well for me until I experienced some signal intermittent drop outs in one of my projects. In this particular project, the electronic keyboard was situated on stage, which was around 17m away from the laptop off stage running Pro Tools for recording midi instruments (from USB cable) and multi-track audio (from Dante Virtual Soundcard). Therefore, maintaining the signal transmission integrity is important as the recording will be used for post-production. In addition, some of the instrument sounds used were from Pro Tools and not the keyboard's internal Digital Sound Processing (DSP) unit.

The 20m USB cable was not usable for the show as it was not providing a stable connection between the laptop and keyboard. Therefore, I needed to find an alternative solution instead of investing my money in a cable that is "guaranteed" to work. To my surprise, an active USB extender cable worked perfectly in this scenario. The standard USB extender cable is passive. It does not boost the signal strength in any way to allow stable transmission longer than the rated 5m. In contrast, an active USB extender has circuitry built in to amplify the signal for longer transmission range.

An alternative option to the conductor based active USB extender cable is an optical based USB cable. The fiber optic USB cables from Corning have impressive length options (10m, 15m, 30m and 50m). The rated length for such cables has far exceeded the norm and it provides viable solutions to individuals who require data transfer over long distances.

Corning 3.0ptical Cable

The dull looking active USB extender cable has proved its usefulness and reliability. As compared to the Corning optical USB cable, the price of your standard active USB cable will cost you around $25 at Sim Lim Square. The cable is not only limited to what I have applied but it is surprisingly versatile in many applications. For instance, I have used it to interface a non-networkable photocopying machine to a computer permanently (around 9m apart). You can even take advantage of the USB 3.0 version to transfer files between devices that are far apart from each other.

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Do cables really matter? (Part 2)

May 15, 2016 by Fu Yongwei in audio, tech

In this post, I would like to focus on my views in buying high-end premium cables for your audio system. As previously mentioned, cables act as a medium to transmit signals between hardware. Hence, a good quality cable will retain signal integrity for the system.

Premium cables such as the Audioquest Angel 3.5mm to RCA cables are easily priced at around $1000 USD for a 1m cable. As a comparison, a similar type of cable from a generic brand such as Cable Matters would cost around $10 USD. That is a huge sum of money to be invested in a single cable and that calls for a more in-depth review to justify purchasing the high-end version.

Left: Audioquest Angel 3.5mm to RCA cable, Right: Cable Matters 3.5mm to RCA cable

When it comes to evaluating the quality of a certain cable, there are a few aspects we can look at. As mentioned in my previous post, a balanced cable is resistant to unwanted interferences even in long cable distances, as the signal will go through a differentiating process to cancel out the induced noise. Regardless of material superiority, whether the cable is being made of gold or copper, an unbalanced cable (such as the 3.5mm to RCA cable) is susceptible to unwanted interference.

Do take note that if a balanced cable is used in an unbalanced output, the cable will still be carrying an unbalanced signal and vice versa. Using the correct type of cable for the right application is therefore important, as it will cause unnecessary problems if not followed through.

Audio cables are passive, meaning there is no active circuitry built inside to improve signal quality. It is a transmitting medium that carries signal from one point to another, where the signal loss will be a factor that affects signal integrity (in this case signal strength) relative to distance. Unless the cable is using some proprietary transmission protocol, premium cables do not improve any signal quality, as it is basically a passive medium.

In addition, even when only premium cables are used in connecting different audio hardware together (e.g. from DA to speakers), one would have to be mindful that the internal wiring of these audio hardware does not utilise these "premium" cables in its circuitry. Thus, the premium cables have pretty much no influence in terms of signal integrity improvement, as the rest of the hardware does not make use of them. Although you can try and redo all the wiring schematics in your beloved audio gears with premium high-end cables, I am fairly sure that this will not have any significant enhancement towards your system performance.

A twisted pair (balanced) cable is almost as good as it gets when it comes to retaining signal integrity, as the twisted pair conductors improve CMRR and the balanced cable is resistant to interferences. The introduction of a Star-Quad Cable was to further improve the immunity to interferences and it has shown real-world improvement. These four conductors shielded configuration can be thought of as two twisted pairs twisted together. Using four small conductors in place of two conductors allows the loop area of the cable to further reduce and improve rejections towards electromagnetic interference. Star-Quad cables seem to be used mainly in live sound applications where long cable runs are common.

As a bonus, LinusTechTip has also reviewed a $1000 HDMI cable recently and the cable does not show any significant improvements in terms of signal quality.

The Wireworld Platinum Starlight 7 HDMI Cable is probably the best thing we've ever seen come through the office. Just kidding...

Investing in good quality cables is important, as their serviceability will easily justify the cost. I will advise not to spend the unnecessary cash in buying high-end premium cables, as it does not have any influence in your signal quality improvement. I would rather save up the money and invest in another important audio gear such as the power conditioner.

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Do cables really matter? (Part 1)

May 12, 2016 by Fu Yongwei in audio, tech

It is very common for people to pay little attention to cable quality when building electronic systems. Cables play a huge role in your system potentials, as they are the medium for transmitting data or power. If the cables are inefficient as a transmitting medium, the performance of the system will suffer as there is a high level of signal losses.

Before we continue with this topic, we must first recognise the difference between a wire and a cable. In simplified terms, a wire is a conductor (e.g. copper) whereas a cable is made up of multiple conductors. For instance, a speakon connector cable will usually have a pair of conductors in order to transmit amplified signals to the respective speaker cabinet.

As a result, good quality cables are definitely important when it comes to building a system. In this post, I would like to address a few considerations for any practitioners building an audio system. All audio cables are screened (shielded), meaning that an outer conductor wraps around the others conductors to shield them from electromagnetic interference. The outer conductor may be made of various materials such as wire braid or metal foil.

In most cases, the outer conductor is connected to ground so that any induced currents (due to interference) will flow directly to ground rather than being mixed with the audio signal. However, this is not a perfect solution, which is why balanced signals were introduced.

In an unbalanced cable, there is a single inner core (conductor) that carries the audio signal while the outer screen act as the ground cable. The screen also doubles as the signal return path, hence any interference induced will cause the audio signals to be mixed with the interfering currents. These unwanted interferences are more prominent in long cable distances or where there are nearby sources of strong interference (e.g. dimming circuits).

Even in long distances, balanced cables are resistant against such interferences. As compared to its counterpart, a balanced cable has two inner conductors, commonly known as positive and negative. Similar to an unbalanced cable, the screen conductor is grounded but no signal will run through this path. Balanced audio transmission relies on Common-Mode Rejection Ratio (CMRR) to cancel out any interference picked up in the cable. A differentiating process will be implemented at the input stage of the receiving end to cancel out any noise. If any interference makes it through the screen conductor, it is likely to have the same influence on both the positive and negative conductors, thus the CMRR will reject any interference common in both conductors (positive and negative).

As mentioned, CMRR is based on the premise that unwanted external interference is being induced into both signal conductors equally, but is there any way to minimise the susceptibility of interference pickups to maximise the benefits of CMRR? Decreasing the distance between the two conductors by twisting them together helps to equalise the coupling effect of the interference with the audio signal.

A "loop antenna" is formed when two conductors have a spaced gap between them. Therefore, the farther apart the two conductors are, the large the antenna will be, thus picking up more interference along the line of transmission. Minimising the area between the two conductors helps to reduce unwanted hum and buzz from this type of interference, which the cable shield is almost ineffective against. The distance between the twists is called the lay of a pair. Shortening the lay by increasing the number of twists will improve not only the CMRR but also cable flexibility. Do take note that by shortening the lay distance, it would require more material (wire) and machine time, thus increasing hardware cost.

Understanding the characteristics of a good quality cable is vital in system building, as it determines the signal integrity carried through the medium. As a rule of thumb, if there is no option for a balanced output, I will usually not have any unbalanced cable of more than 5m, any more than that would require a Direct Injection (DI) box to transform the unbalanced audio to balanced. The above characteristics have been proven to help sustain signal integrity in real world practices and investing in a bunch of good quality cables is certainly wise.

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