I. General Order of Public Broadcast Engineering Design The public broadcasting mentioned here refers to the sound broadcasting of cable transmission, which is usually used in public venues, buildings, and communities, for background music broadcasting, paging broadcasting, and forced insertion of forcible broadcasting. . The design of this type of public broadcasting project is usually carried out in the following order:

1, first of all should consider the selection and configuration of broadcast speakers

2. Selection of broadcast power amplifier

3, broadcast partition

4. Construction of the broadcasting system

Second, the selection and configuration of broadcast speakers

1. Selection of broadcast speakers In principle, broadcast speakers of different specifications should be selected according to the environment.

For example, in a room with a ceiling ceiling, an embedded, back coverless ceiling speaker should be used. Such as OBT-608, OBT-808, OBT-809, OBT-502 and other series. This type of speaker has a simple structure, is relatively inexpensive, and is easy to construct. The main disadvantage is that there is no back cover and it is easy to be bitten by insects and rats.

In indoors with only ceilings and ceilings (such as open-frame shopping malls), ceiling-mounted domes (OBT-208) or ceiling speakers with rear covers (such as OBT-102, OBT-106, etc.) should be used. Since the ceiling is equivalent to an infinite baffle, the use of a speaker without a back cover in the presence of a ceiling does not cause an acoustic short circuit. The situation is quite different when there is no ceiling. If you still use a ceilingless speaker without a back cover, the effect will be very poor. In this case, hoisting speakers should be used in principle. However, if the investment is too large, a ceiling speaker with a rear cover can also be used. The rear cover with the rear cover ceiling speaker not only has the general mechanical protection effect, but also plays a role in preventing the acoustic short circuit to a certain extent.

In ceiling-free rooms (such as underground parking lots), wall-mounted speakers or indoor sound columns should be used. The former is such as OBT-426, OBT-428, OBT-466-468-469, etc.; the latter is such as OBT-121-123, OBT-160-162 and so on.

Outdoors, an outdoor sound column or horn should be used. The former such as OTB-161-163, OBT131-133 and other series; the latter such as OBT-307, OBT-309 and other series. This type of sound column and horn not only has a rainproof function, but also a large volume. Since the outdoor environment is empty and there is no reverberation effect, it is necessary to select a variety with a loud volume.

In gardens and grasslands, grass speakers should be used. Such as OBT-1801, OBT-1802, OBT-1803 and other series. These speakers are rainproof, sleek, and the volume and sound quality are more elegant.

In the halls with exquisite decoration and high ceilings, it is advisable to use mural speakers with elegant shapes and harmonious tones. Such as OBT-3040, OBT-4050, OBT-6090, OBT-60120 and other series.

In the case of high fire protection requirements, fire-resistant speakers should be used. For example, OBT-207, OBT-108 and other series. This type of speaker is fully sealed and its outlet is mated to the flame-retardant sleeve.

As for the choice of brand and grade, nature is related to investment. The OBT series is mainly based on the popular grade to the medium grade.

2, the configuration of the broadcast speaker

In principle, the broadcast speakers are arranged in the broadcast service area in a uniform and decentralized manner. The degree of dispersion should be such that the signal-to-noise ratio in the service area is not less than 15 dB.

Generally, the noise floor of a high-grade office corridor is about 48~52dB, the noise floor of a superstore is about 58~63dB, and the noise floor of a busy section is about 70~75dB. Considering the accident, the scene may be very confusing, so for the need of emergency broadcasting, even if the broadcasting service area is an office building, the noise floor should not be estimated too low. For this reason, as a general consideration, in addition to the bustling and lively places, it is possible to roughly consider the noise floor as 65~70dB (except in special cases). According to this calculation, the sound pressure level of the broadcast coverage area should be above 80~85dB.

Since broadcast speakers are typically distributed, the sound pressure level of the broadcast coverage area can be approximated as a contribution from a single broadcast speaker. According to the relevant electroacoustic theory, the sound pressure level SPL of the speaker coverage area has the following relationship with the sensitivity level LM of the speaker, the electric power P fed to the speaker, and the distance r between the listening point and the speaker:

SPL=LM+10lgP–20lgrdB(1)

The sensitivity level of the ceiling speaker is between 88~93dB; the rated power is 3~10W. At a frequency of 90 dB/8 W, the sound pressure level at 8 m from the speaker is about 81 dB. The above calculations do not consider the contribution of the early reflection sound group. Indoors, the contribution of early reflections and adjacent speakers can increase the sound pressure level by about 2~3dB.

According to the above approximate calculation, in the venue where the ceiling is not higher than 3m, the ceiling speakers can be evenly arranged at a distance of 5~8m. If only background music is considered without considering emergency broadcast, the distance can be increased to 8~12m. In addition, the fire accident design and installation specification (hereinafter referred to as 'specification') applicable to mainland China has the following rigid rules: “The number of speakers in public places such as walkways, halls, restaurants, etc. should be guaranteed from any part of this floor. Recently, the walking distance of a speaker is no more than 15m. Speakers should be provided at the intersection of the walkway and at the corner. The last speaker at the end of the walkway is no more than 8m away from the wall.

There is basically no early reflection sound group in outdoor places, and the effective coverage of a single broadcast speaker can only take the lower limit calculated above. Since the distance corresponding to the lower limit is very short, a sound column composed of a plurality of speakers should be used in principle. For each doubling of the signal electrical power fed to a group of speakers (eg, a sound column) (provided the group is acceptable), the sound pressure level can be increased by 3 dB. Please pay attention to the meaning of "double". It is doubled from 1 to 2; it is doubled from 2 to 4. In addition, for every 1 time increase in distance, the sound pressure level will drop by 6dB. According to the above rules, it is not difficult to estimate the arrangement distance of the outdoor sound column. For example, taking the OBT-1127 outdoor sound column as an example, its rated power is 40W, which is more than 4 times that of a single ceiling speaker. Therefore, its effective coverage distance is more than twice that of a single ceiling speaker. In fact, this distance can be larger. Because the sensitivity of the column is higher than that of a single ceiling speaker (about 3~6dB), the distance can be doubled for every 6dB increase. In other words, the coverage distance of the OBT-1127 sound column can reach more than 20m. However, the radiation angle of the sound column is relatively narrow, and it is effective only about 60 to 90 degrees (horizontal angle) in front of it. The specific calculation can still be obtained by equation (1).

Third, the selection of broadcast power amplifier

Broadcast amplifiers are different from HI-FI amplifiers. Its main feature is its 70V and 100V constant voltage output terminals. This is because broadcast lines are usually quite long and must be transmitted with high voltage to reduce line losses.

The most important indicator of a broadcast amplifier is the rated output power. How much rated output power should be used, depending on the total power of the broadcast speakers. For the broadcast system, as long as the total power of the broadcast speaker is less than or equal to the rated power of the power amplifier, and the voltage parameters are the same, it can be arbitrarily matched, but considering the line loss, aging and other factors, the power margin should be left as appropriate. According to the requirements of the 'Specifications', the capacity of the power amplifier equipment (equivalent to the rated output power) should generally be calculated as follows:

P=K1?K2?ΣP0(2)

P—Amplifier output total electric power (W)

P0—Maximum electric power for each split (equivalent to partition) simultaneous broadcast

P0=Ki?PiPi—i-zone speaker rated capacity

Ki—The i-th partition requires coefficients at the same time:

Service broadcast room program, take 0.2~0.4

Background music system, take 0.5~0.6

Business broadcast, take 0.7~0.8

Fire accident broadcast, take 1.0

K1—Line attenuation compensation coefficient: 1.26~1.58

K2 - Aging coefficient: 1.2~1.4

Therefore, if it is a background music system, the rated output power of the broadcast power amplifier should be about 1.3 times the total power of the broadcast speaker.

However, all public address systems should, in principle, be able to carry out emergency broadcasts of disasters. Therefore, the system must be equipped with an emergency broadcast amplifier. According to the 'Specification' requirements, the rated output power of the emergency broadcast power amplifier should be 1.5 times the sum of the speaker capacity in the three partitions with the largest broadcast speaker capacity. As for other specifications of the broadcast power amplifier, it depends on the specific structure and investment of the broadcast system.

Fourth, the broadcast partition

A public address system is usually divided into several areas, with managers (or pre-programs) deciding which areas must broadcast, those areas to be suspended, and those areas to be inserted into emergency broadcasts.

The partitioning scheme depends in principle on the needs of the customer. Generally refer to the following rules:

1. Buildings are usually divided into floors. Shopping malls and amusement parks are usually divided by departments. Sports venues are usually divided into stands. Residential quarters and resorts are usually managed by property management.

2. The administrative department and the public place should be divided into districts.

3. The volume of important departments or broadcast speakers must be arbitrarily adjusted by field personnel. In short, the partition is for ease of management. Any part that needs to be treated separately should be divided into different zones. However, in each zone, the total power of the broadcast speakers should not be too large and must be compatible with the capacity of the partitioner and amplifier. Taking the OBT-8010 partitioner in the OBTPA series as an example, the power capacity of each zone is 1000 VA, but the total capacity of 10 zones should not exceed 10000 VA. Thus, if 10 zones are operating at full capacity, the average zone should not exceed 1000 VA (approximately 1000 W).

V. Construction of the broadcasting system

1. Simple system, a public address system

At least the following links must be configured:

Broadcast speakers, broadcast amplifiers, preamps, microphones.

A. The simplest scheme is shown in Figure 1.

Microphone

The OBT-6N series of broadcast amplifiers have built-in preamplifiers (commonly known as "mergers"). The minimum power of this series is 60W, which can drive 8~16 ceiling speakers or 5~10 columns (depending on the model of the speaker and the sound column); the maximum power is 350W, and its driving ability is 6 times that of the former.

This simple system can only publish voice broadcasts such as notifications, pages, and speech. If you want to broadcast background music, broadcast news, and release recordings, you can add CDs, decks, tuner (radio) and other devices.

The OBT-6N50 Series has multiple line input connectors that can be fully connected to these devices.

The OBT-6N series can also be equipped with multiple microphones for use by small and medium-sized assembly podiums. The main microphone has a priority function, and its signal can suppress other inputs (make it silent) to force the insertion of a priority speech or an urgent broadcast.

B. Another simple scheme is shown in Figure 2.

Microphone

The OBT-6060 radio amplifier is an integrated amplifier with built-in radio. It is fully functional and can be used in one machine. Its main microphone also has priority and can be used for emergency insertion and remote control with a remote control.

The rated output of the OBT-6060 is 60W.

The common shortcoming of the above two simple systems is that there is no linkage interface with the fire center. As a typical public address system, the above links and interfaces are required, as detailed below.

2, the minimum system

The minimum system refers to a system in which the public broadcasting function is basically complete. The recommended scheme is shown in Figure 3.

Compared with the simple system, it mainly increases the partition link, timing control link, alarm link and the interface with the fire center. Normally, the system runs under the management of the programmable timer OBT-9100 (the power of the relevant link is turned on and off according to the pre-programmed program), and the punctual signal of the schedule is played on time. When the fire center sends an alarm signal to the system, the relevant link is forcibly started through the linkage interface (regardless of the state of the program); at the same time, all the partitions are forcibly cut into the emergency broadcast, regardless of whether it is in the closed state.

Secondly, in the figure, the power amplifier and the preamplifier are also separated, and the combination and control of the system are more convenient; in addition, a diagnostic device is configured to monitor the operation of the system.

3, partition alarm / forced insertion

Although the minimum system has partitioning and intrusion functions, its intrusion function is not ideal. First, the alarm cannot be released in a partition. Once an alarm occurs, all partitions enter the alarm state at the same time. This is appropriate for a small system. For example, in a primary school, regular broadcasting is necessary (at least the classroom and office should be treated separately), and the alarm should of course be released at the same time. But for larger systems, it's not appropriate, and a full release of alerts can be confusing. Second, the alarm can force open those partitions that are normally closed, but cannot open those partitions that have been closed by the live sound controller.

A. In order to realize the zone alarm, there must be two power amplifiers. The configuration is shown in Figure 4 (only the part related to the zone alarm is drawn). In Figure 4, the background program and the alarm signal are sent to the A and B ends of the partitioner respectively. When the alarm occurs, the alarm signal only enters the alarm zone, while the other zones play the background music as usual.

B. In order to forcibly open (bypass) the sound controller, there are two modes. One is called the three-wire system (Figure 5), and the other is called the four-wire system (Figure 6).

The configuration of the three-wire three-wire system is shown in Figure 5.

As can be seen from Figure 5, the three-wire system features only three terminal wirings - N, R, and C. The illustration shows the background music status. During the alarm, the broadcast signal line is switched to the alarm channel by the partitioner in the system center (computer room), and the 24V power supply (called the strong plug power supply) is sent by the system center to drive the intrusion control relay to make the R line short-circuit with the N line. The purpose is to bypass the tone controller. However, the sound controller used here must be compatible with the three-wire system, and the applicable devices are WH-1 and WH-2 (the latter is used for the multi-program source system). When a sound controller controls a group of speakers, an expander (EH series) must be added due to the large controlled power. As for the intrusion control relay, the applicable device is the CH-1F series; the strong insertion power supply can be OBT-8060.

It is necessary to point out that some users can easily misconnect the R line in the three-wire system to the output of the emergency broadcast power amplifier. As a result, the emergency broadcast crosses the background music broadcast. In fact, the R line is just a bypass-like wire that is switched by the sound controller when driven by an emergency broadcast command. There is no independent emergency broadcast signal line in the three-wire system, and the N line is managed by the partitioner (OBT-8011). Normally used for background music; the emergency partitioner OBT-8011 automatically switches it to the emergency broadcast amplifier output for emergency broadcast.

The configuration of the four-wire four-wire system is shown in Figure 6.

As can be seen from Figure 6, there are four four-wire terminal wirings - C, N, +24V, -24V. Compared with the three-wire system, the difference is that the thrust relay is placed inside the sound controller. Applicable audio controllers are OBT-10NF and OBT-10NFS.

4. The typical system recommendation scheme is shown in Figure 7.

Compared with the minimum system, the typical system adds alarm matrix, partition intrusion, partition paging, and telephone.