sangoma vs digium 回声抑制模块对比

很多人在疑问,sangoma 是否真的比digium 强吗?

In recent months, both [url=]Digium[/url] and [url=]Sangoma[/url] have released T1/E1 Interface cards for use with [url=]Asterisk Open Source PBX[/url] that provide hardware Echo Cancellation.
Echo can negatively impact QOS (Quality of Service) on a VOIP calland lead to an unsatisfactory user experience. There are two sources ofecho on voice communication networks. The first, and most common causeof echo is impedance mismatches anywhere in the circuit-switched [url=]phone[/url]network. When telco cabling is spliced or terminated in connectors, orin the conversion of 4-wire phone circuits to 2 wires, a discontinuityoccurs that causes an impedance mismatch on the phone circuit. Thegreater the extent of impedance mismatch, the more leakage of transmitaudio on the receive side of the line when it terminates on a 2-wirephone circuit or device. The main cause of this impedance mismatch isthe 2 to 4 wire hybrid that converts a 4-wire audio path to a local2-wire loop. This tip-and-ring loop is the most-used type of telephoneconnection made to subscribers from your telephone company centraloffice, and directly drives common analog telephone devices.
Acoustic coupling between the microphone and speaker of a telephonedevice leads to Acoustic echo, the second most common cause of echo onvoice communication networks. Received caller audio can often “leak” tothe microphone in cheaper speakerphones and hands-free cellular phones,because of sound pressure from a speaker or earpiece device
This inherent echo led to the development of Echo Cancellers, inorder to improve the quality of voice communications. Echo cancellersare very complex digital signal processing devices, and the algorithmsthat drive them are produced by a handful of technically proficientcompanies. Naturally, as telecommunications evolves from the legacyPSTN network to the [url=]VOIP[/url] network, echo cancellation is still a very necessary component for ensuring QOS.
[url=]Digium[/url] offers hardware echo cancellation on their [url=]TE406P[/url] and [url=]TE411P[/url] telephony boards, as well as their new [url=]TDM2400[/url] series full length analog PCI cards. [b]Digium’shardware echo cancellation provides 64ms across 32 channels; however,when it scales over 32 channels it is reduce to 16ms per channel acrossall channels. // 注意 这里是 16 ms 了,功能马上下来了。
Sangoma offers hardware echo cancellation on their [url=]A104D[/url]Quad T1 card, as well as on their as yet unreleased series of analogtelephony boards. Unlike Digium where available ms of echo cancellationdecreases as active channel density increases,[b] Sangoma hardware echo cancellation provides a full 128ms of echo cancellation on all channels, regardless of density.[/b]
[b]// 无论呼叫量多么大,还是128ms, 说明你买的是货真价实的东西。
By providing hardware echo cancellation which extends to a full128ms on all channels at full load, Sangoma provides a more robustcompensation for echo which is more effective under extreme conditions.The echo tail length represents how long the effect of an echo extendsafter the time of the echo source. If a ping is transmitted at timezero, 16ms (16 ms = 128 taps at 8 samples per millisecond) of tail willdeal with any echo from that ping that occurs within 16 ms. The echoitself dies away in about 8ms, so as long as the delay between the pingand the start of the echo is less than 8ms, then the echo cancellerwill work well. The problem arises when the signals get delayed. Forinstance traveling through one Telco switch adds about 5ms of two-waydelay, so it is easy to see how a bit of extra switching could delaythe echo right out of an echo canceller’s
The above image illustrates the echo from a “ping” on a real line where there are
switching delays. In cases of extreme echo as pictured above, normal128 tap echo cancellation would be ineffective at cancelling the echopresent on the line.
Software echo cancellation works quite well for electrical echoesthat are within the 16ms/128 tap range. Where hardware echocancellation really has benefit is where the delays are longer. TheSangoma echo canceller is a carrier-grade device with 1024 taps (128ms)of tail to handle the most severe echo problems.


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