In this commentary, I am going to explain the spec “total harmonic distortion”, often also referred to as “THD” which is often used in order to show the quality of wireless speakers shown at this hyperlink.
Wireless loudspeakers are available in all different shapes and sizes. Selecting the right type may often be difficult . I will shed some light on a frequently used spec which is utilized to publish the technical quality of the speaker: “total harmonic distortion” or “THD”. THD is generally not as easily understood as a few other commonly utilized specs including “signal-to-noise ratio” or “frequency response”. Briefly, “harmonic distortion” shows how much the music signal is being degraded as a result of the speaker or in other words how much the signal differs from the original signal. There are two common methods in order to articulate harmonic distortion, either in percent (%) or in decibel (dB). A -20 dB or 10% distortion means that one 10th of the radiated audio is a consequence of distortion whilst -40 dB or 1% would mean that one percent of the energy are harmonic products of the original signal. Though, be careful because there are in fact several elements which cause harmonic distortion. Cordless loudspeakers along with any type of active loudspeaker or active subwoofer all have built-in power amplifiers in order to drive the loudspeaker element. The amp itself will have a specific level of distortion. Typically the distortion of the amplifier will be bigger the more output power it supplies to the loudspeaker. In general producers are going to publish amplifier distortion based on a specific level of output power, normally a lot less than the rated highest amp output power.
Distortion ratings for various power levels are usually specified for a few output power levels or as a diagram listing distortion versus output power. Both of these methods allow to better evaluate the quality of the amplifier.Furthermore, please note that distortion generally is measured for a certain test tone frequency. Commonly a 1 kHz sine wave tone is utilized during the measurement. This allows comparing equipment from various vendors. However, distortion normally varies with different frequencies. Many digital amps are going to exhibit growing distortion amid higher frequency which can not easily be discovered through glancing at the specification sheet.
One more component causing distortion is the speaker element that generally works with a diaphragm that carries a voicecoil that is suspended in a magnetic field. The voicecoil will track the magnetic field which is controlled by the audio signal in order to move the diaphragm. Yet, this movement is not completely linear. As such there will be speaker element distortion which additionally depends on the amount of power with which the speaker is driven.
The total distortion of the speaker thus is the sum of the amp distortion along with the speaker element distortion. Moreover, there are other contributing factors. The loudspeaker enclose will vibrate to some extent and thus add to the distortion.
To figure out the overall distortion of a speaker, a signal generator is utilized which provides an ultra-linear signal to the loudspeaker and also a measurement microphone that is attached to an audio analyzer to calculate the amount of harmonics radiated by the loudspeaker. However, pure sine signals barely give an accurate picture of the distortion of the wireless speaker with real-world signals. A better distortion analysis is the so-called intermodulation distortion analysis where a test tone that includes several sine waves is used. Next the level of harmonics at different frequencies is measured.
One more factor contributing to distortion is the signal transmission of wireless loudspeakers, particularly with products which broadcast an analog signal at 900 MHz. Digital audio transmission on the other hand has the least level of audio distortion. The signal is digitized in the transmitter and not affected throughout the transmission itself. Digital cordless audio transmitters are available at 2.4 GHz plus 5.8 GHz.
An ever expanding quantity of cordless gadgets which include wireless outdoor speakers available at amphony.com/products/wireless-speaker.htm is bringing about growing competition for the precious frequency space. I am going to check out a number of technologies which are utilized by the latest digital sound systems in order to see how well these solutions can work in a real-world situation. The popularity of cordless gizmos just like wireless speakers is mainly responsible for a quick rise of transmitters which transmit in the preferred frequency bands of 900 MHz, 2.4 Gigahertz and 5.8 GHz and thus cordless interference has turned into a serious issue.
The least expensive transmitters normally broadcast at 900 MHz. They work similar to FM radios. Since the FM signal uses a small bandwidth and thus only occupies a tiny part of the free frequency space, interference may be prevented by changing to a new channel. The 2.4 GHz and 5.8 Gigahertz frequency bands are used by digital transmitters and also are getting to be quite crowded lately because digital signals take up much more bandwidth as compared to analog transmitters.
Just changing channels, on the other hand, is no dependable solution for avoiding certain transmitters which use frequency hopping. Frequency hoppers like Bluetooth gadgets as well as several cordless telephones are going to hop throughout the whole frequency spectrum. As a result transmission on channels is going to be disrupted for brief bursts of time. Audio can be viewed as a real-time protocol. Consequently it has strict requirements regarding dependability. In addition, small latency is critical in several applications. For that reason more sophisticated strategies are required to ensure reliability. One of these methods is called forward error correction or FEC for short. The transmitter is going to transmit extra data besides the audio data. The receiver uses an algorithm which utilizes the extra information. In the event the signal is damaged during the transmission as a result of interference, the receiver can easily remove the erroneous data and restore the original signal. This approach works if the amount of interference won’t go beyond a specific threshold. FEC is unidirectional. The receiver doesn’t send back any kind of information to the transmitter. As a result it is frequently employed for systems like radio receivers in which the number of receivers is large. Yet another method utilizes receivers that transmit data packets to the transmitter. The transmitters incorporates a checksum with every data packet. Every receiver can determine whether a specific packet has been received correctly or damaged because of interference. Subsequently, each cordless receiver sends an acknowledgement to the transmitter. Since lost packets must be resent, the transmitter and receivers have to store information packets in a buffer. Using buffers brings about a delay or latency in the transmission. The amount of the delay is proportional to the buffer size. A larger buffer size increases the stability of the transmission. A big latency can be a problem for several applications however. Particularly if video exists, the audio should be synchronized with the movie. Furthermore, in multichannel surround sound applications in which a number of loudspeakers are cordless, the wireless speakers should be synchronized with the corded speakers. One limitation is that systems in which the receiver communicates with the transmitter can usually just transmit to a few wireless receivers. Also, receivers have to incorporate a transmitter and generally consume additional current
Often a frequency channel may become occupied by a different transmitter. Preferably the transmitter is going to understand this fact and change to another channel. To do this, a number of wireless speakers continuously check which channels are available so that they can quickly switch to a clean channel. Since the transmitter has a list of clear channels, there is no delay in trying to find a clear channel. It’s simply picked from the list. This technique is frequently referred to as adaptive frequency hopping spread spectrum.