electronic hearing aid has at minimum a microphone, a loudspeaker (commonly called a receiver), a battery, and electronic circuitry. the electronic circuitry varies among devices, even if they are the same style. the circuitry falls into three categories based on the type of audio processing (analog or digital) and the type of control circuitry (adjustable or programmable). hearing aid devices generally do not contain processors strong enough to process complex signal algorithms for sound source localization. analog analog audio may have: adjustable control: the audio circuit is analog with electronic components that can be adjusted. the hearing professional determines the gain and other specifications required for the wearer, and then adjusts the analog components either with small controls on the hearing aid itself or by having a laboratory build the hearing aid to meet those specifications. after the adjustment the resulting audio does not change any further, other than overall loudness that the wearer adjusts with a volume control. this type of circuitry is generally the least flexible. the first practical electronic hearing aid with adjustable analog audio circuitry was based on us patent 2,017,358, "hearing aid apparatus and amplifier" by samual gordon taylor, filed in 1932. programmable control: the audio circuit is analog but with additional electronic control circuitry that can be programmed by an audiologist, often with more than one program. the electronic control circuitry can be fixed during manufacturing or in some cases, the hearing professional can use an external computer temporarily connected to the hearing aid to program the additional control circuitry. the wearer can change the program for different listening environments by pressing buttons either on the device itself or on a remote control or in some cases the additional control circuitry operates automatically. this type of circuitry is generally more flexible than simple adjustable controls. the first hearing aid with analog audio circuitry and automatic digital electronic control circuitry was based on us patent 4,025,721, "method of and means for adaptively filtering near-stationary noise from speech" by d graupe, gd causey, filed in 1975. this digital electronic control circuitry was used to identify and automatically reduce noise in individual frequency channels of the analog audio circuits and was known as the zeta noise blocker. digital block diagram of digital hearing aid digital audio, programmable control: both the audio circuit and the additional control circuits are fully digital. the hearing professional programs the hearing aid with an external computer temporarily connected to the device and can adjust all processing characteristics on an individual basis. fully digital circuitry allows implementation of many additional features not possible with analog circuitry, can be used in all styles of hearing aids and is the most flexible; for example, digital hearing aids can be programmed to amplify certain frequencies more than others, and can provide better sound quality than analog hearing aids. fully digital hearing aids can be programmed with multiple programs that can be invoked by the wearer, or that operate automatically and adaptively. these programs reduce acoustic feedback (whistling), reduce background noise, detect and automatically accommodate different listening environments (loud vs. soft, speech vs. music, quiet vs. noisy, etc.), control additional components such as multiple microphones to improve spatial hearing, transpose frequencies (shift high frequencies that a wearer may not hear to lower frequency regions where hearing may be better), and implement many other features. fully digital circuitry also allows control over wireless transmission capability for both the audio and the control circuitry. control signals in a hearing aid on one ear can be sent wirelessly to the control circuitry in the hearing aid on the opposite ear to ensure that the audio in both ears is either matched directly or that the audio contains intentional differences that mimic the differences in normal binaural hearing to preserve spatial hearing ability. audio signals can be sent wirelessly to and from external devices through a separate module, often a small device worn like a pendant and commonly called a "streamer", that allows wireless connection to yet other external devices. this capability allows optimal use of mobile telephones, personal music players, remote microphones and other devices. with the addition of speech recognition and internet capability in the mobile phone, the wearer has optimal communication ability in many more situations than with hearing aids alone. this growing list includes voice activated dialing, voice activated software applications either on the phone or on the internet, receipt of audio signals from databases on the phone or on internet, or audio signals from television sets or from global positioning systems. the first practical, wearable, fully digital hearing aid was invented by maynard engebretson, robert e morley, jr. and gerald r popelka. their work resulted in us patent 4,548,082, "hearing aids, signal supplying apparatus, systems for compensating hearing deficiencies, and methods" by a maynard engebretson, robert e morley, jr. and gerald r popelka, filed in 1984. this patent formed the basis of all subsequent fully digital hearing aids from all manufacturers, including those produced currently. the signal processing is performed by the microprocessor in real time and taking into account the individual preferences of the user (for example, increasing bass for better speech perception in noisy environments, or selective amplification of high frequencies for people with reduced sensibility to this range). the microprocessor automatically analyzes the nature of the external background noise and adapts the signal processing to the specific conditions (as well as to its change, for example, when the user goes outside from the building). in speech enhancement, for example using neural networks, finds application in hearing aids. problems may arise if these methods filter out emergency sounds such as fire alarms and car horns. difference between digital and analog hearing aids analogue hearing aids make all the sounds picked up by the microphone louder. for example, speech and ambient noise will be made louder together. on the other hand, digital hearing aid (dha) technology processes the sound using digital technology. before transmitting the sound to the speaker, the dha microprocessor processes the digital signal received by the microphone according to an algorithm. this allows certain-frequency sounds to be made louder according to the individual user's settings (personal audiogram), and the dha can automatically adjust to various environments (noisy streets, quiet room, concert hall, etc.). for users with varying degrees of hearing loss, it is difficult to perceive the entire frequency range of external sounds. dhas with multi-channel digital processing allow a user to "compose" the output sound by fitting a whole spectrum of the input signal into it. this gives users with limited hearing abilities the opportunity to perceive the whole range of ambient sounds, despite the personal difficulties of perception of certain frequencies. moreover, even in this "narrow" range the dha microprocessor is able to emphasize desired sounds (e.g. speech), lowering unwanted loud, high, etc., sounds at the same time. according to research dhas have a number of significant advantages compared to analogue hearing aids: "self-learning" and adaptive adjustment. they can implement adaptive selection of amplification parameters and processing. effective acoustic feedback reduction. the acoustic whistling common to all hearing aids can be adaptively controlled. effective use of directional microphones. directional microphones can be adaptively controlled. extended frequency range. a larger range of frequencies can be implemented with frequency shifting. flexibility in selective amplification. they can provide more flexibility in frequency specific amplification to match the individual hearing characteristics of the user. improved connection to other devices. connection to other devices such as smartphones and televisions is possible. noise reduction. they can reduce the background noise level to increase user comfort in noisy environments. speech recognition. they can distinguish the speech signal from the overall spectrum of sounds, which facilitates speech perception. these advantages of dhas were confirmed by a number of studies relating to the comparative analysis of digital hearing aids of second and first generations and analog hearing aids. difference between digital hearing aids and hearing aid applications smartphones have all the necessary hardware to perform the functions of a digital hearing aid: microphone, ad converter, digital processor, da converter, amplifier, and speakers. external microphone and speakers can also be connected as a special headset. the operational principles of hearing aid applications correspond to general operational principles of digital hearing aids: the microphone perceives an acoustic signal and converts it to digital form. sound amplification is achieved through hardware and software in accordance with the user's hearing characteristics. then, the signal is converted to analog form and received in the headphones by the user. the signal is processed in real time. stereo headsets with two speakers can be used, which allows separate binaural hearing correction for the left and right ear. unlike digital hearing aids, the adjustment of hearing aid applications is an integral part of the application itself. hearing aid applications are adjusted in accordance with the user's audiogram. the whole adjustment process is automated so that the user can perform audiometry on their own. the hearing correction application has two modes: audiometry and correction. in the audiometry mode, hearing thresholds are measured. in the correction mode, the signal is processed with respect to the obtained thresholds. hearing aid applications also provide for different computational formulas for the calculation of sound amplification based on the audiometry data. these formulas are intended for maximum comfortable speech amplification and best sound intelligibility. hearing aid applications allow the user to save different user profiles for different acoustic environments. thus, in contrast to the static settings of digital hearing aids, the user can quickly switch between the profiles depending on the acoustic environment. one of the most important characteristics of the hearing aid is acoustic feedback. in hearing aid applications, there is a significant hardware delay, so hearing aid applications use a signal processing scheme with the minimum possible algorithmic delay to make it as short as possible. difference between psap and digital hearing aids personal sound amplification products (psap) are classified by the fda as "personal sound amplification devices". these compact electronic devices are designed for people without hearing loss. unlike hearing aids (which the fda classifies as devices to compensate for hearing impairment), the use of psap does not require a medical prescription. such devices are used by hunters, naturalists (for audio observation of animals or birds), ordinary people (for example, to increase the volume of the tv in a quiet room), etc. psap models differ significantly in price and functionality. some devices simply amplify sound. others contain directional microphones, equalizers to adjust the audio signal gain and filter noise. in modern days, some people refer to these devices as otc hearing aids. evolution of hearing aid applications there are audio players designed specifically for the hard-of-hearing. these applications amplify the volume of the reproduced audio signal in accordance with the user's hearing characteristics and act as a music volume amplifier and assistive hearing aid. the amplification algorithm works on the frequencies that the user hears worse, thus restoring natural hearing perception of the sound of music. just as in hearing aid applications, the player adjustment is based on the user's audiogram. there are also applications that not only adapt the sound of music but also include some hearing aid functions. such applications include a sound amplification mode in accordance with the user's hearing characteristics as well as a noise suppression mode and a mode allowing the user to hear ambient sound without pausing the music. also, some applications allow the hard-of-hearing to watch video and listen to the radio with comfort. the operational principles of these applications are similar to those of hearing aid applications: the audio signal is amplified on the frequencies that the user hears worse. hearing aid adaptation a person using a hearing aid for the first time often cannot make use of all its advantages quickly. the structure and characteristics of hearing aids are thoroughly devised by specialists in order to make the adjustment period as simple and quick as possible. however, despite this, a beginning hearing aid user certainly needs time to get used to it. the process of adjusting to hearing prostheses consists of the following steps: initial adjustment of the device fine adjustments adaptation to the new sound due to the plasticity of the central nervous system, inactive hearing centers in the brain's cortex switch over to processing auditory stimuli in