Multichannel registration systems.

Multi-channel recording systems.

General Provisions

Recently, thanks to the improvement of computer technology (increased processor performance, the emergence of high-capacity “hard” drives, magneto-optical drives, etc.), as well as advances in the development of analog-to-digital converters (ADCs), a large number of multi-channel recording systems (MCS) have begun to appear on the special equipment market) audio information (mainly telephone conversation recording systems).

The bulk of recording systems present on the Russian market are based on personal computers, but there are devices made in the form of separate structurally and functionally complete systems, such as digital multi-channel tape recorders from EYRETEL, Great Britain.

Figure 1 shows the functional diagram of the MCR, which is common to all devices, the characteristics of which are given in Table 1.

Input analog signals Lin.1 — Lin. N (from a telephone line or another source) are sent to the interface unit, which ensures matching with the input signal source in terms of level, resistance, etc., as well as preliminary filtering of the signal.

Then the signals are sent to the input signal digitization board. This board is installed directly in the computer slot. Most often, on the ISA bus slot.

The number of channels that one input board can digitize and transmit to a computer depends on the characteristics of a specific product, but usually this number is a multiple of two, i.e. 2,4,8,16, etc.

Fig. 1

Usually all multi-channel systems have a control output, which is used to listen to the recordings made, to which you can connect headphones or speakers. Some systems also have a line output.

Some important characteristics of multichannel recording systems:

Digital data flow density — depends on the quality of digitization of the recorded signal and determines the required volume of the “hard” disk, magneto-optics or other information carriers used for archiving. Measured in MB/hour.

Note. The required disk volume is determined as:

Vd[MB] = Pw[MB/hour] * Ta[hour], where

Pw is the flow density, Ta— total time of conversations on all channels.

When recording telephone conversations, the standard is the quality of the channel in which the analog-to-digital conversion is performed by the pulse-code modulation method with a logarithmic companding characteristic, 8-bit data format and a quantization frequency of 8 kHz (for an operating frequency band of 300-3400). This digital stream corresponds to a density of 28.8 MB/hour.

For example, with such a density, only about 36 hours of recordings can be made on a 1 GB disk.

Data compression.

Almost all of the systems listed in Table 1 use some compression algorithms to reduce the density of the data flow and, accordingly, to increase the total time of recorded conversations.

However, the downside of using data compression is the deterioration of the quality of the recorded signals.

The most common algorithm that allows for acceptable quality with a fairly high compression ratio (four times) is adaptive differential pulse — code modulation ( ADPCM ), which provides a recording density of 7.2 MB/hour.

Other algorithms are also used, such as adaptive delta modulation (ADM ), but with the same compression ratio they provide slightly worse quality than ADPCM.

Connecting to a telephone line.

Typically, MSR interface units provide the ability to connect to telephone lines in parallel (similar to connecting a parallel telephone set), which is the simplest and most convenient for users.

In order for MSR sensors not to affect the monitored line, their input resistance for alternating current should be more than 10 kOhm. Some systems have the ability to connect to the line in parallel or in series.

Storing information.

Almost all MSRs use a PC “hard” disk to store information, recording onto which is carried out in the “ring” mode, which implies entering new information in place of the oldest.

For long-term archiving of information, many MSRs, in addition to “hard” disks, also include magneto-optical storage devices, onto which the required volume of already recorded information can be dumped.

Types of recording initiation.

Since the purpose of the MSR is mainly to record telephone conversations, then in order to save disk space, it is most advisable to record at the start of the conversation.

There are two main methods of starting used in the MSR — acoustic start and start by an external signal.

In the acoustic start mode, recording is turned on when the signal exceeds a certain threshold, which is set by the user.

The software of many MSRs allows you to adjust the trigger threshold separately for each channel. The signal that occurs on the line when the telephone handset is picked up is usually used as an external trigger signal.

Additional service.

Networking.

Some MSRs have the ability to increase resources by networking, and the number of channels being registered can increase almost unlimitedly.

Warning signal.

Issued by the MSR to warn subscribers who are talking that they are being recorded.

Conversion to WAV format.

Many MSRs record data to disk in their own format.

In order for the user to be able to listen to the recordings he needs on any other computer using a regular sound card (Sound Blaster), the MSR software package includes converters that convert files in the internal MSR format to the WAV format, which is playable by all sound cards.

2. MSR overview.

As can be seen from Table 1, MSRs are multifunctional devices, so a good way to evaluate a particular system would be a generalized indicator that includes such components as channel price, functionality, and recording quality.

Table 1

“ + “ — yes, “ — “ — no, n/a — no data; s.b. PC — PC system unit.

* — indicates the volume recommended by the manufacturer. To select the volume you need, see the note to Chapter 1.

** — is determined by dividing the cost of the max. number of channels by the number of channels. The cost of the PC is not taken into account.

Table 1. Continued

“ + “ — yes, “ — “ — no, n/a — no data; PC system unit — PC system unit.

* — indicates the volume recommended by the manufacturer. To select the volume you need, see the note to Chapter 1.

** — is determined by dividing the cost of the max. number of channels by the number of channels. The cost of the PC is not taken into account

*** — this system is not supplied without a PC. The characteristics and design of the PC are determined by the manufacturer.

**** — power is supplied from a 12V battery.

Table 1. Continued

“ + “ — yes, “ — “ — no, n/a — no data; s.b. PC — PC system unit.

* — indicates the volume recommended by the manufacturer. To select the volume you need, see the note to Chapter 1.

** — is determined by dividing the cost of the max. number of channels by the number of channels. The cost of the PC is not taken into account.

Table 1. End

“ + “ — yes, “ — “ — no, n/a — no data;s.b. PC — PC system unit.

* — indicates the volume recommended by the manufacturer. To select the volume you need, see the note to Chapter 1.

** — is determined by dividing the cost of the max. number of channels by the number of channels. The cost of the PC is not taken into account.

Unfortunately, MSR manufacturers do not provide such an important parameter as loss of intelligibility in the recording path—playback in the specifications of their devices., although almost all systems use speech signal compression, so the quality of a particular system can only be assessed indirectly, by the digital stream density and compression algorithm.

Table 2 allows you to compare the MSRs according to the above generalized criterion.

Note: The functional capabilities are indicated in conventional units on a twenty-point scale.

Table 2.

The comparison results allow us to first note the “Stelth Line systems” (the “Grand Prix” company) and “MAG-M” (the “Signatek” company), which have a wide range of functional capabilities and have a relatively low cost per channel.

However, this is true for the case when the user purchases a system with the maximum number of channels; when purchasing a system with a small number of channels, the cost can increase significantly.

The “Stelth Line” system series includes a wide range of models — from “Stelth Line Mini” (6 channels) to “Stelth Line Big Professional”, which allows recording up to 64 channels. In the multi-user functional mode for the “Stelth Line Big Professional” system, the number of recording channels is not limited.